Scale: make change in window selection work across monitors

When a window is made selected, e.g., when zoomed, make sure that
the active monitor is updated, too.

This addresses a bug which may cause the wrong window (a window on another monitor)
to be keyboard-closed after zooming.
Merge pull request #2422 from autarkper/scale-dual-monitor-selected-fix

Scale: make change in window selection work across monitors