Having everything lumped together made this a maintenance hassle because it affected how the level has to be stored.
This hasn't been tested yet, so it may not work as intended!
currentUILevel is now primaryLevel.
For ZScript, currentVMLevel was added. This is also exported as 'level' and will change as needed.
This also means that no breaking deprecations will be needed in the future, because in order to sandbox a level only 4 variables need to be handled: level, players, playeringame and consoleplayer.
The remaining global variables are not relevant for the level state.
The static 'level' has been mostly removed from the code except some places that still need work.
Two files were split:
g_level.h contained both the game data definitions and some prototypes belonging to the game logic. These were split up.
decallib.cpp contained both the data and the animation thinkers. The thinkers are now in their own file.
