Fix PanelConfigView being shown on the wrong screen in multi-monitor setups

Original commit message(s)

• Fix PanelConfigView being shown on the wrong screen in multi-monitor setups

Plasma::Corona::availableScreenRectChanged is emitted for all screens, we need to check if it ours.

Reason for the change

In multi-monitor setups panel configuration popup can be shown on the different screen from the panel (non-deterministically since the order of availableScreenRectChanged signals may be different).

Test plan

I checked that popup now always shown on the correct screen.

Screenshots or screen recordings

Before	After

Bugs fixed

I wasn't able to find a related bug report (should I create one?).