Created ReplayAsset interface and AssetRepository class which can contain such ReplayAssets. ReplayAssets allow users to add custom Files (so far only images, might be extended by .obj files) to the Replay which will later be used for CustomImageObjects.

Created GuiReplayManager to allow users to manage their assets.
Created BoundingUtils class to provide a simple method to fit an image into given bounds. This implementation fixes https://trello.com/c/WTnicWkJ/

src/main/java/eu/crushedpixel/replaymod/gui/GuiAssetManager.java

@@ -0,0 +1,175 @@
+package eu.crushedpixel.replaymod.gui;
+
+import eu.crushedpixel.replaymod.assets.AssetFileUtils;
+import eu.crushedpixel.replaymod.assets.AssetRepository;
+import eu.crushedpixel.replaymod.assets.ReplayAsset;
+import eu.crushedpixel.replaymod.gui.elements.*;
+import eu.crushedpixel.replaymod.gui.elements.listeners.FileChooseListener;
+import eu.crushedpixel.replaymod.gui.elements.listeners.SelectionListener;
+import eu.crushedpixel.replaymod.utils.MouseUtils;
+import net.minecraft.client.gui.GuiScreen;
+import net.minecraft.client.resources.I18n;
+import org.lwjgl.opengl.GL11;
+import org.lwjgl.util.Point;
+
+import java.awt.*;
+import java.io.File;
+import java.io.FileInputStream;
+import java.io.IOException;
+
+public class GuiAssetManager extends GuiScreen {
+
+    private String screenTitle;
+
+    private boolean initialized = false;
+    private GuiEntryList<ReplayAsset> assetGuiEntryList;
+    private GuiAdvancedButton removeButton;
+    private GuiAdvancedTextField assetNameInput;
+    private GuiFileChooser fileChooser, addButton;
+
+    private ComposedElement inputElements;
+
+    private ComposedElement composedElement;
+
+    private AssetRepository assetRepository;
+
+    private ReplayAsset currentAsset;
+
+    public GuiAssetManager(AssetRepository assetRepository) {
+        this.assetRepository = assetRepository;
+    }
+
+    @Override
+    public void initGui() {
+        if(!initialized) {
+            screenTitle = I18n.format("replaymod.gui.assets.title");
+
+            assetGuiEntryList = new GuiEntryList<ReplayAsset>(fontRendererObj, 0, 0, 0, 0);
+            addButton = new GuiFileChooser(0, 0, 0, I18n.format("replaymod.gui.add"), null, AssetFileUtils.getAllAvailableExtensions()) {
+                @Override
+                protected void updateDisplayString() {
+                    this.displayString = baseString;
+                }
+            };
+
+            addButton.addFileChooseListener(new FileChooseListener() {
+                @Override
+                public void onFileChosen(File file) {
+                    try {
+                        ReplayAsset newAsset = assetRepository.addAsset(file.getName(), new FileInputStream(file));
+                        assetGuiEntryList.addElement(newAsset);
+                    } catch(Exception e) {
+                        e.printStackTrace();
+                    }
+                }
+            });
+
+            removeButton = new GuiAdvancedButton(0, 0, 0, I18n.format("replaymod.gui.remove")) {
+                @Override
+                public void performAction() {
+                    assetRepository.removeAsset(currentAsset);
+                    assetGuiEntryList.removeElement(assetGuiEntryList.getSelectionIndex());
+                }
+            };
+
+            assetNameInput = new GuiAdvancedTextField(fontRendererObj, 0, 0, 150, 20);
+            assetNameInput.hint = I18n.format("replaymod.gui.assets.namehint");
+
+            fileChooser = new GuiFileChooser(0, 0, 0, I18n.format("replaymod.gui.assets.changefile"), null, new String[0]) {
+                @Override
+                protected void updateDisplayString() {
+                    this.displayString = baseString;
+                }
+            };
+            fileChooser.addFileChooseListener(new FileChooseListener() {
+                @Override
+                public void onFileChosen(File file) {
+                    if(currentAsset == null) return;
+                    try {
+                        currentAsset.loadFromStream(new FileInputStream(file));
+                    } catch(IOException e) {
+                        e.printStackTrace();
+                    }
+                }
+            });
+
+            fileChooser.width = 150;
+
+            inputElements = new ComposedElement(assetNameInput, fileChooser);
+            composedElement = new ComposedElement(assetGuiEntryList, addButton, removeButton, inputElements);
+
+            inputElements.setEnabled(false);
+
+            assetGuiEntryList.addSelectionListener(new SelectionListener() {
+                @Override
+                public void onSelectionChanged(int selectionIndex) {
+                    currentAsset = assetGuiEntryList.getElement(selectionIndex);
+                    inputElements.setEnabled(currentAsset != null);
+
+                    assetNameInput.setText(currentAsset != null ? currentAsset.getDisplayString() : "");
+                    fileChooser.setAllowedExtensions(currentAsset != null ? AssetFileUtils.fileExtensionsForAssetClass(currentAsset.getClass()) : new String[0]);
+                }
+            });
+        }
+
+        int visibleEntries = (int)Math.floor(((double)this.height-(45+20+15+20))/14);
+
+        assetGuiEntryList.width = 150;
+        assetGuiEntryList.xPosition = width/2 - assetGuiEntryList.width - 5;
+        assetGuiEntryList.setVisibleElements(visibleEntries);
+
+        assetGuiEntryList.yPosition = assetNameInput.yPosition = 45;
+
+        addButton.xPosition = assetGuiEntryList.xPosition-1;
+        addButton.width = 77;
+        removeButton.width = 76;
+        removeButton.xPosition = addButton.xPosition+addButton.width;
+
+        addButton.yPosition = removeButton.yPosition = assetGuiEntryList.yPosition+assetGuiEntryList.height+2;
+
+        assetNameInput.xPosition = fileChooser.xPosition = this.width / 2 + 5;
+        fileChooser.yPosition = assetNameInput.yPosition + 20 + 5;
+
+        initialized = true;
+    }
+
+    @Override
+    public void drawScreen(int mouseX, int mouseY, float partialTicks) {
+        this.drawCenteredString(fontRendererObj, screenTitle, this.width / 2, 5, Color.WHITE.getRGB());
+
+        int leftBorder = 10;
+        int topBorder = 20;
+
+        drawGradientRect(leftBorder, topBorder, width - leftBorder, this.height - 10, -1072689136, -804253680);
+
+        composedElement.draw(mc, mouseX, mouseY);
+
+        if(currentAsset != null) {
+            int y = fileChooser.yPosition + 20 + 5;
+            int height = (addButton.yPosition+addButton.height)-y;
+
+            GL11.glColor4f(1, 1, 1, 1);
+            currentAsset.drawToScreen(fileChooser.xPosition, y, 150, height);
+        }
+
+        super.drawScreen(mouseX, mouseY, partialTicks);
+    }
+
+    @Override
+    protected void mouseClicked(int mouseX, int mouseY, int mouseButton) throws IOException {
+        composedElement.mouseClick(mc, mouseX, mouseY, mouseButton);
+        super.mouseClicked(mouseX, mouseY, mouseButton);
+    }
+
+    @Override
+    protected void keyTyped(char typedChar, int keyCode) throws IOException {
+        Point mousePos = MouseUtils.getMousePos();
+        composedElement.buttonPressed(mc, mousePos.getX(), mousePos.getY(), typedChar, keyCode);
+        super.keyTyped(typedChar, keyCode);
+    }
+
+    @Override
+    public void updateScreen() {
+        composedElement.tick(mc);
+    }
+}