display.c: Re-set the button grabs on the frame xwindow as well as

the client window, when the preferences change.

Fixes the following:
- Assign "Special key to move and resize windows" to Alt (windows settings)
- Launch nemo, note that alt-drag on anything moves the window, and is never
  received by nemo to initiate DnD (This is correct).
- Assign special key to Disabled (windows settings)
- Attempt to alt-drag on an icon or list item, note that this *should*
  initiate an "Ask"-type DnD with the selection icon.  Instead, nothing
  happens.

The frame's xwindow was never having its grabs updated, so it would
continue to intercept whatever the initial grab was for the window,
for the lifetime of the window.

src/core/display.c

@@ -5339,6 +5339,12 @@ prefs_changed_callback (MetaPreference pref,
         {
           MetaWindow *w = tmp->data;
           meta_display_ungrab_window_buttons (display, w->xwindow);
+
+          if (w->frame)
+            {
+              meta_display_ungrab_window_buttons (display, w->frame->xwindow);
+            }
+
           meta_display_ungrab_focus_window_button (display, w);
           tmp = tmp->next;
         }
@@ -5361,6 +5367,11 @@ prefs_changed_callback (MetaPreference pref,
             {
               meta_display_grab_focus_window_button (display, w);
               meta_display_grab_window_buttons (display, w->xwindow);
+
+              if (w->frame)
+                {
+                  meta_display_grab_window_buttons (display, w->frame->xwindow);
+                }
             }
           tmp = tmp->next;
         }