Added more robust uri and uri part resolution for assets.
Snapshot of a roughly working JSON style applier thingy
And another step done
Add style states
Make UIButton compatible with new UI system
Adapt GUI system to recent asset manager changes by Immortius
Implement UIWindow#reload
Add Tuple2 class
Refactor method to add a unit-test asset source directory from prefabtest
Add layout definition test
applyTo needs be invoked after setModal and maximize

Otherwise, it would not be possible to change the size or
the "modality" of a window from within the style, because those
settings would get overwritten by the subsequent calls to
setModal and maximize.
Add all elements from the UIMenuMain to the main menu layout def
Add WIP UIController support (events currently fail to be invoked though)