Removed 'default' argument from collision_layer EnumProperty declaration. It wasn't doing anything and errored the plugin in Blender 2.8.

Changed collision layer to be game dependent, rather than relying on Oblivion collision layers. No Skyrim functionality - best fix for that would be pyffi/nif.xml update.

Removes UI properties
Stores them as actual materials (proper names pending until nifxml transition is settled)

Capsules visualize and export correctly again. Also add skyrim havok material property to UI.

If each class is imported by the parent __init__ of a module then it
will automatically call the classmethod register for any classes that
are imported.

Not sure if this is actually needed as object.py doesn't have one and is
registered correctly. Should look into it for general registration.

Allows for improved maintainability & extensibility.