Blender Git Loki

UV: Extend edge selection support

- Add edge selection support for operators: mouse select, box select,
circle select, lasso select, (de)select all, invert selection and
select more/less
- Flush selections between UV verts and edges
- Fix: prevent deselection of neighbouring edges when deselecting an
edge in sticky location + edge select mode

Fix: Prevent deselection of surrounding UV faces

With face + sticky loc/vertex mode, trying to deselect UV faces would
sometimes result in surrounding UV faces being deselected as well. This
commit fixes that allowing conditional deselection of shared UV
vertices based on the selection state of surrounding UV faces.

UV: Edge selection support - Initial

* Add UV edge selection flag - MLOOPUV_EDGESEL
* Refactor existing UV element selection functions to use the edge
selection flag wherever required
* Refactor existing UV element check functions to ensure proper
selection states using the edge selection flag
* Refactor UV select all operator to use edge selection flag
* New functions for selecting vertices or edges that share the same
location, either on 3D mesh or in UV space.
* Add small penalty for finding the nearest UV edge. Ensures that UV edge
selection will select other edges sharing the same location in
successive selection attempts.
* Expose UV edge selection flag as boolean in Python

Differential Revision: https://developer.blender.org/D12028