Summary:
Hiding the old menu after the new menu is shown isn't possible on Wayland:
```
qt.qpa.wayland: setGrabPopup called with a parent, QtWaylandClient::QWaylandXdgSurface(0x17dd550) which does not match the current topmost grabbing popup, QtWaylandClient::QWaylandXdgSurface(0x120d9f0) According to the xdg-shell protocol, this is not allowed. The wayland QPA plugin is currently handling it by setting the parent to the topmost grabbing popup. Note, however, that this may cause positioning errors and popups closing unxpectedly because xdg-shell mandate that child popups close before parents
```

Test Plan:
Before, Wayland: Trying to open a menu under Wayland while one is already open fails.
After, Wayland: Trying to open a menu under Wayland while one is already open succeeds.
Ensure no regressions on X11.

Reviewers: #plasma, #kwin, broulik

Reviewed By: #plasma, broulik

Subscribers: broulik, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D29464