screen edge: Emit cleanup signals if needed on teardown
The Effects API has one signal screenEdgeApproaching with effects monitoring the last signal to stay in sync.
If an Edge is destroyed whilst visible, effects currently do not get notified and it can stay there forever.
This patch emits screenEdgeApproaching if appropriate with a factor of 0 when an edge is destroyed.
BUG: 403354 (cherry picked from commit 3ea70174)