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.
UI always runs on the primary level, so this does not need the ability to operate on multiple levels. Additionally, this can later be set to null when running play code so that scope violations result in an abort.
Doing this to Menu itself would not work because the different menus require different parameters.
This also means that all menus that are routed through menu items must inherit from either ListMenu, OptionMenu or GenericMenu.
All other types can only be used internally.
This should complete the menu scriptification.
