Update availableScreenRect when a panel moves to another screen

Summary: When disconnecting the primary screen on a dual monitor setup and reconnecting it back, the available size of the secondary screen desktop may be reduced to the thickness of the primary screen panel, as can be seen by dragging a rubber band or trying to move some widgets. This apparently happens because the QML bindings had last been updated at the short moment when both panels were temporarily placed on the same screen. Update availableScreenRect therefore when a panel changes its screen.

Reviewers: Plasma, davidedmundson

Reviewed By: Plasma, davidedmundson

Subscribers: broulik, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D29831