Also a general cleanup of CMakeLists.txt and many small code changes to satisfy Clang that complained quite a lot and actually wouldn't compile the binary at all at first.
- Refactoring System Environment data
- Added Virtual System Manager class
- Added "all", "favorites" and "last played" systems
- Added GuiInfoPopup class for notifications
- Added Favorites to metadata, as well as a shortcut to toggle favorites
- Added warning if enabling systems but themes don't support it
- Added "filter by favorites" per system
- Adjusted "Go to Random Game" behavior to account for the fact that we now have an "All Games" system
- Added "sort by system name" for the collections
