Fix Klipper empty prevention on wayland
requested to merge ngraham/plasma-workspace:backport-c6d52910ebc1fb1abd89d286aec9c24ca102db77 into Plasma/5.27
Manual cherry-pick of !2908 (merged) because it didn't cherry-pick cleanly to make sure it doesn't break tests or whatevs
When an application terminates, we receive a signal for the Selection and restore it. While restoring it, we also receive a signal for the Clipboard, but don't handle it since we're currently locked due to the Selection.
This patch fixes that by creating separate locks for Selection and Clipboard.
BUG: 468422 (cherry picked from commit c6d52910)