Provide better options/defaults for what UV's are submitted to Xatlas

[mattedicksoncom]

Currently relies on a UV index for all objects working on the assumption that generally UV's will be ordered like:

1. Main user made UV's
2. Secondary UV's
3. Lightmap UV's

Possibly add option to grab main UV by name, or to purposely avoid lightmap UV's by name

[mattedicksoncom]

4a61e32 Provides a reasonably flexible choice for deciding what UV's to use.
I'm happy with it for now but will keep open as things can always improve