tray/na-xembed: Destroy socket window when ending the embedding

tray/na-xembed: Destroy socket window when ending the embedding

Currently when embedding a tray icon is ended we are leaving the X11
socket window around as we don't really destroy it, while we're just
listening for its child destruction.

So, when embedding is ending we need to make sure that:
 - If the plug window is still alive, move it back to its old parent
   (normally that's the root window, but we query for it, to be sure)
 - If the plug window has been already destroyed or reparented (by us or
   by something else), we've to just destroy the socket window

Closes: https://gitlab.gnome.org/GNOME/gnome-shell/-/issues/7614
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/3818>
(cherry picked from commit cd39f7ab4725588d237e7b79763384c0e92f41ff)