[applet/appmenu] Fix top level menu hovering state when QMenu is open

This bug may happen if you rapidly move mouse horizontally and clicking
on the menu at the same time: in some cases the originally pressed menu
item would remain visually pressed (because hovered property can't reset
while grab is active), and some other item whose index matches the value
of Plasmoid.nativeInterface.currentIndex will be `down` too.

FIXED-IN: 5.26