Make appmenu work on wayland

Make appmenu work properly on Wayland

The toplevel menu is positioned using the plasmashell protocol.
For QtWayland to create popups we need a input serial. To prevent being
killed by the compositor, an optional parameter is added to the DBus
method to pass the serial of the event that triggered showing the appmenu.
BUG:430662
FIXED-IN:5.23

---

Drop input grabbing hack

Apparently this was introduced to prevent the window losing focus if
https://bugs.kde.org/show_bug.cgi?id=310476 is to be believed. However
on X the window does not lose focus without it.
Furthermore it actually improves keyboard navigation on X, which after a
child menu was closed stopped working.
On Wayland this prevents things working at all.
BUG:420980
FIXED-IN:5.23