AutoUV: New options for packing UVs

[micheus]

I have been working on adding some new options to pack UV islands, and it's getting promising.

In addition to the two new options, I also made the internal option available in case we mess around with the UV placement and want to restart.

I have also added an option to move only the selected ones, so we can place the main one(s) the way we want and then let the algorithm distribute the rest.
Reference | Legacy Bounding Box | Polygon


mages:
1) Initial displacement;
2) Wings3D's internal (legacy method)
3) Bounding Box (new method)
4) Polygon (new method)

The legacy one (2) brings a new result for every new round (that is interesting sometimes because we can try different arrangement easily);
The bounding box option would word better for islands with a more regular shape;
The last one (4) is the most expensive, but give us the better packing result.

If accepted for inclusion in the next release I'm going to let here a little guide: Pack user guide