Unit/feature model importing and def creation

[gajop]

The purpose of this is to allow inexperienced users to get objects in the game easier.

This may be difficult because there are many unit def systems, and there probably shouldn't be a standard we enforce.
It should still rely on text editing of unit defs, as to not place restrictions on the unit def format (and use the full benefits of scripted unit defs), but there should be a clear (clickable) link between unit (visual) and its def.
It should allow loading of models (e.g. .dae files), texture files and basic/empty unit/feature defs. This is the key component really.