Allows to quickly select alternate elems in a path (matching checker-deselect options).
- adds redo support to MESH_OT_shortest_path_pick, allowing for other options.
- de-duplicates code between 2x path select operators.
- expose 'Topology Distance' property for path picking.
- remove unused 'extend' property.
When mixing vert/edge/face with select-visible,
face selection could fail when not close enough to the center.
This also fixes a bug where the bias for verts over edges would
prefer faces over edges too, making edges harder to pick.
Mixing edge with other selection modes works more predictably now.
- Ctrl+RMB only worked for edges & faces
- Menu item 'Select Shortest Path' only worked for vertices.
Now Ctrl+RMB works for vertices and the menu item works for verts/edges/faces (depending on the current selection).
