Collision layer support

[Shfty]

Currently the only way to configure collision layers is via scripts attached to custom SolidClass entries. This means that collision layer handling has to be accounted for in all custom SolidClasses, which is undesirable - this feature is global enough in Godot that it should be treated the same in Qodot, so a feature needs to be added to account for it.

Initial idea: Add a pair of global properties that behave similarly to _phong, but send through collision layer / mask bitmasks instead. Worldspawn layers are also an option, but that's a more rigid solution than custom classes. Textures aren't a good solution since they're per-face rather than per-brush.