display: Add 'closing' signal

Emitted when the MetaDisplay is closing. Meant for clean up that depends
on things that will be torn down during closing.

https://gitlab.gnome.org/GNOME/mutter/merge_requests/912

src/core/display.c

@@ -156,6 +156,7 @@ enum
   SHOWING_DESKTOP_CHANGED,
   RESTACKED,
   WORKAREAS_CHANGED,
+  CLOSING,
   LAST_SIGNAL
 };
 
@@ -493,6 +494,12 @@ meta_display_class_init (MetaDisplayClass *klass)
                   G_SIGNAL_RUN_LAST,
                   0, NULL, NULL, NULL,
                   G_TYPE_NONE, 0);
+  display_signals[CLOSING] =
+    g_signal_new ("closing",
+                  G_TYPE_FROM_CLASS (klass),
+                  G_SIGNAL_RUN_LAST,
+                  0, NULL, NULL, NULL,
+                  G_TYPE_NONE, 0);
 
   g_object_class_install_property (object_class,
                                    PROP_FOCUS_WINDOW,
@@ -971,6 +978,8 @@ meta_display_close (MetaDisplay *display,
 
   display->closing += 1;
 
+  g_signal_emit (display, display_signals[CLOSING], 0);
+
   meta_compositor_unmanage (display->compositor);
 
   meta_display_unmanage_windows (display, timestamp);