UV Island relax algorithm and usage overhaul

Blender's two standard UV algorithms "ABF+" and "LSCM" are bread and butter for UV unwrapping but over the years there have been more efficient additional ways of dealing with organic UVs. Modo's "adaptive" relax algorithm for example does a superb job in relaxing UV islands that might need more cuts in Blender in order to be unwrap with little distortion. Another nice addition to the whole thing would be if the relax command could be invoked over already unwrapped UV islands iteratively. Basically like the UV relax command but instead of making thins worse the algorithm actually makes it better. So the relax command should not just smooth out the polygons in the UV editor by themselves but actually reiterate over the islands relative to the mesh.

Think for eample of these steps:

• You set our seams and unwrap, getting an okayish result
• You run a different UV relax metod over your UV islands and achieve a little less stretching (let's say you chose a different type of relax) but are getting some overlaps.
• so you sculpt, push and rotate your UVs to a better distribution but gained some really ugly, uneven stretching.
• so instead of running "smooth" and maybe getting even more (albeit more regular) distortion over the whole island you simply invoke the relax algorithm with a low iteration over the current layout. That way you can pull your uneven distribution closer to the actual meshflow and achieve middle ground with probably way better UVs without tons of cuts.

This is an example with Angle based unwrapping left and 500 iterations over the same unwrap with the "adaptive" algorithm to the right.

So my proposal here is:

1. Add more and more current relax algorithms
2. enable to iterate an actual relax over the UV islands instead of just "smooth" which does more bad than good, in most cases