This patch adds edge selection support for UV editing (refer T76545).
Developed as a part of GSoC 2021 project - UV Editor Improvements.
Previously, selections in the UV editor always flushed down to vertices
and this caused multiple issues such as T76343, T78757 and T26676.
This patch fixes that by adding edge selection support for all UV
operators and adding support for flushing selections between vertices
and edges. Updating UV select modes is now done using a separate
operator, which also handles select mode flushing and undo for UV
select modes. Drawing edges (in UV edge mode) is also updated to match
the edit-mesh display in the 3D viewport.
Notes on technical changes made with this patch:
* MLOOPUV_EDGESEL flag is restored (was removed in rB9fa29fe7652a).
* Support for flushing selection between vertices and edges.
* Restored the BMLoopUV.select_edge boolean in the Python API.
* New operator to update UV select modes and flushing.
* UV select mode is now part of editmesh undo.
TODOs added with this patch:
* Edge support for shortest path operator (currently uses vertex path logic).
* Change default theme color instead of reducing contrast with edge-select.
* Proper UV element selections for Reveal Hidden operator.
Reviewed By: campbellbarton
Differential Revision: https://developer.blender.org/D12028
ffaaa0bcbf