Add some form of level editing

[mokiat]

It is currently not possible to easily model any new assets and levels. It might be time to add some form of level editing, be it quite primitive at first.

• Allow for conversion of Blender models to game models. (the glTF format is a good candidate for conversion)
• Allow assignment of collsiion boxes and spheres to models.
• Allow arrangement of models onto a level scene.
• Allow skinned meshes (opt)