Merge branch 'master' into soc-2021-uv-editor-improvements-edge-selection

Edge selection support for Edge ring and loop select operator

* Adds edge selection support for edge ring and edge loop select operator
* Selecting face loops in the UV editor is now done through the loop
select operator

Cleanup and fix: UV Invert selection

* Code and comment cleanup
* Fix: Make invert selection respect sticky modes when using edge select
mode

Fix: Check edge selection only for common vertex

Allow edge selection test only for UV edges that share a common UV
vertex. Previous logic didn't check if the UV edges shared the same
UV vertex.

UV: Edge selection for UV select pinned operator

Ensure edge selection for UV select pinned operator by flushing the
selection upwards (verts to edge)

UV: Refactor UV select split operator

Refactor UV select split operator to use MLOOPUV_EDGESEL flag

Merge branch 'master' into soc-2021-uv-editor-improvements-edge-selection

Fix: Invert selection and free unreleased memory

- Free unreleased memory from vertex map
- Select Invert operator now works with UV face select mode as well

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