* moved the sprite renaming out of the file system entirely into a caller-provided callback.
* renamed several functions to closer match the terms of a file system.
* moved the VM interface out of the implementation.
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.
This is supposed to be come the place where all pure play code should be placed, but for that all CVARs and CCMDs and other things that do not directly handle play data should be taken out to make code reviewing easier. These now get collected in two separate files, g_cvars.cpp and g_dumpinfo.cpp respectively.
The sole ZScript property in here has also been moved - to thingdef_properties.cpp.
