Revert "Revert "Prevent panel going out of screen boundaries""

This reverts commit 17774bc4c6

There is still a known culprit (duplicate display names) so the hack shouldn't be reverted.

CCBUG: 353975
CCBUG: 438114


(cherry picked from commit 9c7ac7061c)

shell/panelview.cpp

@@ -859,6 +859,9 @@ void PanelView::moveEvent(QMoveEvent *ev)
     updateEnabledBorders();
     m_strutsTimer.start(STRUTSTIMERDELAY);
     PlasmaQuick::ContainmentView::moveEvent(ev);
+    if (m_screenToFollow && !m_screenToFollow->geometry().contains(geometry())) {
+        positionPanel();
+    }
 }
 
 void PanelView::keyPressEvent(QKeyEvent *event)