Asset manager, refactoring, cleanup, loading screen.

Added a static asset manager, used to load and hold all game assets
(graphics, fonts, ui, later sound). With that, also added a loading
screen which loads all the assets at startup. For now, it just displays
a message, but a progress bar is possible at a later time.

Also cleaned up the code a bit, added comments in some places, removed
all logging code, etc.

All assets are now loaded centraly and can be acquired as needed via
Assets.manager.get(assetName).