xwayland: Init DnD on every Xwayland start

Currently, meta_xwayland_shutdown_dnd() is called from the handler
on_x11_display_closing() triggered from the signal "x11-display-closing"
hooked up from meta_xwayland_init_display().

Once the signal has been triggered, on_x11_display_closing() removes the
signal handler, disconnecting from the signal.

As meta_xwayland_init_display() is called from meta_display_new() which
is issued only once, the signal handler is not restored again.

As a result, meta_xwayland_shutdown_dnd() is not called anymore after
Xwayland has been restarted, but meta_xwayland_init_dnd() will check and
assert that the manager's DND object is NULL.

Basically, restarting Xwayland more that once will trigger an assertion
failure in mutter. That's even more of a problem with autoclose-xwayland
where Xwayland is expected to terminate when there is no meaningful X11
client remaining, which can happen multiple times during the lifetime
of a user session.

To make sure that meta_xwayland_init_display() is called for every new
instance of Xwayland, simply keep the signal hooked in place by not
disconnecting it when triggered.

This reverts commit 9a10b8ff94.

Even though, originally, this issue was first introduced with commit
b4fe1fdd95 ("xwayland: Make setup/teardown
a bit more symmetrical") which didn't actually kept 'x11-display-setup'
and 'x11-display-closing' connected.

Closes: https://gitlab.gnome.org/GNOME/mutter/-/issues/2168
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2339>

src/wayland/meta-xwayland.c

@@ -1067,9 +1067,6 @@ on_x11_display_closing (MetaDisplay         *display,
   g_signal_handlers_disconnect_by_func (meta_monitor_manager_get (),
                                         monitors_changed_cb,
                                         NULL);
-  g_signal_handlers_disconnect_by_func (display,
-                                        on_x11_display_closing,
-                                        manager);
 }
 
 static void