Implement pinned with select-sync (technically not a bug),
more an oversight in !138197.
Some subtle functional changes have been made.
- Select pinned now only works in vertex select mode
since previously it was possible to select vertices in edge/face modes
where the selection wasn't displayed.
- The island selection option is ignored when selecting pinned.
- The select pinned operator wasn't working with select sync edge/face
modes. Exits with an error instead.
Ref !148181
2808c411d7