cleanup: Use g_clear_signal_handler() where possible

This is inspired by 98892391d7 where the usage of
`g_signal_handler_disconnect()` without resetting the corresponding
handler id later resulted in a bug. Using `g_clear_signal_handler()`
makes sure we avoid similar bugs and is almost always the better
alternative. We use it for new code, let's clean up the old code to
also use it.

A further benefit is that it can get called even if the passed id is
0, allowing us to remove a lot of now unnessecary checks, and the fact
that `g_clear_signal_handler()` checks for the right type size, forcing us
to clean up all places where we used `guint` instead of `gulong`.

No functional changes intended here and all changes should be trivial,
thus bundled in one big commit.

https://gitlab.gnome.org/GNOME/mutter/merge_requests/940

src/wayland/meta-wayland-xdg-shell.c

@@ -496,8 +496,8 @@ meta_wayland_xdg_popup_unmap (MetaWaylandXdgPopup *xdg_popup)
 
   if (xdg_popup->parent_surface)
     {
-      g_signal_handler_disconnect (xdg_popup->parent_surface,
-                                   xdg_popup->parent_surface_unmapped_handler_id);
+      g_clear_signal_handler (&xdg_popup->parent_surface_unmapped_handler_id,
+                              xdg_popup->parent_surface);
       xdg_popup->parent_surface = NULL;
     }