Fix activity swtiching through UserActions menu

On wayland the code to keep windows on the current activity whilst the
window was broken was completely broken in porting. We only held the
block for the duration of the method even though the popup remained open
for longer.

On X11, when removing the window from the current activity it would not
update correctly and remain visible. The code path was as follows:
 - the menu is shown an event loop is started
 - we change the activities
- as we close the menu focus changes and workspace calls
UserActionsMenu::close this unsets m_window
- We then never call the blockActivityUpdates(false) at the end of
UserActionsMenu::show

This patch addresses both at once.

We get rid of the nested event loop as that was always evil. This means
slotWindowOperation no longer needs to be queued.

We perform cleanup of m_window and the activity blocker when the menu
closes which should be safer.

BUG: 456873


(cherry picked from commit a7520b36)