Rough Animation Exporting and Basis for Model Export UI

[Sage-of-Mirrors]

This PR implements the ability to export actions on an armature to BCK files. It works for the most part, but there are still some issues:

• Some animations have unexplained jittering. Example: Great Fairy from TWW's entrance animation; see how the upper-right shoulder shrugs back and forth near the end. That's not part of the original animation, and it isn't visible in Blender either.
• There was a bug where negative rotation values would be interpreted as their positive counterparts, e.g. -90 degrees was treated as +270 degrees. This was seemingly fixed by f1ee381, but I didn't test it beyond the custom animation that was giving me trouble. See the incorrect animation vs the correct animation.
• Hermite tangents are not implemented because I couldn't quite grasp how to do them properly. Custom animations on existing rigs should work fine, but re-exporting existing animations will cause them to lose that tangent data.

Despite these caveats, animation exporting should be usable at a basic level in most cases.

This PR additionally contains an export operator and UI panels for BMD/BDL exporting, but as model exporting is not yet implemented I have commented them out of the export menu for now.