Screen Edge bindings for Scripts and Scripted Effects

ScreenEdge is changed to emit a signal whenever a screen edge
got activated without an action or effect taking care of it.

A Script can reserve one to many callbacks for an edge and the
callback get's triggered whenever the signal is emitted. On
deconstruction of the Script the edge is unreserved again.

FEATURE: 299275
FIXED-IN: 4.9.0
REVIEW: 104904