Changes

Created AssetFileUtils to provide valid File Extensions for various ReplayAsset implementations. This class has to be updated if new ReplayAsset implementations are added.

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/