Add Pinning / Masking

[jlampel]

Pinning was a helpful feature in v3, and it allowed the user to mask areas in a more permanent way than the current selection. We also had the ability to use seams as pins.

I don't think pinning is critical for an initial v4.0 release, so I'll add it as a target to save for v4.1. Now that Blender supports custom edge attributes, we could allow the user to choose any attribute, not just seams, as an additional custom mask.