GuiElement interface's mouseClick method returns a boolean which is handled by ComposedElement to prevent mouse clicks on multiple elements at once

ComposedElement's entries are sorted so that GuiDropdown are always drawn on top
Modified thrown UnsupportedOperationExceptions in CustomImageObject to not be thrown if asset is null

src/main/java/eu/crushedpixel/replaymod/gui/elements/GuiAdvancedCheckBox.java

@@ -27,8 +27,8 @@ public class GuiAdvancedCheckBox extends GuiCheckBox implements GuiElement {
     }
 
     @Override
-    public void mouseClick(Minecraft mc, int mouseX, int mouseY, int button) {
-        mousePressed(mc, mouseX, mouseY);
+    public boolean mouseClick(Minecraft mc, int mouseX, int mouseY, int button) {
+        return mousePressed(mc, mouseX, mouseY);
     }
 
     @Override