screenedges: Allow triggering actions from corners on multi-screen setups

Users can now activate corner actions independently on each screen in multi-monitor environments.

The original design followed Fitts’s Law, assuming that inter-screen corners were difficult to trigger. However, with pointer barrier support, this limitation no longer applies.

See also: https://old.reddit.com/r/kde/comments/185kwsa/screen_edges_on_multi_monitor/