Currently the UV editor doesn't support edge selection, edge selection just selects an de-selects the vertices.
Supporting this involves:
- Adding back the edge selection flag (was removed rB9fa29fe7652a: Cleanup: remove MLOOPUV_EDGESEL)
- Add support drawing selected edges.
- Add support for flushing selection between vertices and edges.
- Add support for edge selection for:
- Box select.
- Circle Select.
- De-Select All
- Select Invert.
- Select Mode/Less.