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/resources/assets/replaymod/lang/en_US.lang

@@ -257,7 +257,7 @@ replaymod.input.resettilt=Reset Camera Tilt
 replaymod.input.playpause=Play/Pause Replay
 replaymod.input.marker=Add Event Marker
 replaymod.input.pathpreview=Toggle Path Preview
-replaymod.input.addassets=Add Assets
+replaymod.input.assetmanager=Open Asset Manager
 
 #Keyframe Presets GUI
 replaymod.gui.keyframerepository.title=Keyframe Repository
@@ -357,10 +357,10 @@ replaymod.gui.ingame.unnamedmarker=Unnamed Event Marker
 replaymod.gui.clearcallback.title=Clear all Keyframes?
 replaymod.gui.clearcallback.message=This Callback can be disabled in the Replay Settings.
 
-#Asset Adder Gui
+#Asset Manager Gui
 replaymod.gui.assets.title=Asset Manager
-replaymod.gui.assets.filechooser=Image File
+replaymod.gui.assets.filechooser=Asset File
 replaymod.gui.assets.defaultname=New Asset
 replaymod.gui.assets.emptylist=No Assets available
 replaymod.gui.assets.namehint=Asset Name
-replaymod.gui.assets.backvisible=Visible from behind
+replaymod.gui.assets.changefile=Change Asset File