Use new mesh paths.
Updated models.
If no parts in submodel, enable all subentities.

I.e. make it easier to define models. If a submodel is added, but no
parts are defined, it's expected that this means that all subentities
should be shown.
Added ability to create a Model from a Mesh.
Updated with new assets.

Some older assets were removed, and there are new ones. One change is
that I've tried to match one model per mesh, instead of cramming
multiple meshes into single models. This ought to make the structure
simpler.
Updated with new material names.