Re-implement GLOBAL support

[MattFiler]

Asset editor no longer supports GLOBAL textures/models (note: models aren't actually used in retail, but the engine has support for it).

Should re-implement the ability to load GLOBAL stuff, or should perhaps instead make OpenCAGE automatically update all levels to point away from GLOBAL. The latter seems a bit hacky, but will make stuff easier in the long run as syncing levels and the GLOBAL folder will get tricky between mods.