Replace per UV map selection with a single UV selection for all UV's.
This uses the same data as UV sync select, meaning that it's no longer
possible to keep a different selection when sync-select is disabled.
There is a minor improvement to functionality - previously not possible
to de-select a single face surrounding by selected faces.
Now this is possible because true face selection is supported.
The selection from the active UV-map is converted to use the shared
selection data.
Ref !147523
Co-authored-by: Hans Goudey <hans@blender.org>
f4308aa2d0