main: Notify about shutdown before tearing down

This will allow the shell to tear things down more gracefully.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1349>

src/shell-global.c

@@ -120,6 +120,7 @@ enum
 {
  NOTIFY_ERROR,
  LOCATE_POINTER,
+ SHUTDOWN,
  LAST_SIGNAL
 };
 
@@ -507,6 +508,13 @@ shell_global_class_init (ShellGlobalClass *klass)
                     0,
                     NULL, NULL, NULL,
                     G_TYPE_NONE, 0);
+  shell_global_signals[SHUTDOWN] =
+      g_signal_new ("shutdown",
+                    G_TYPE_FROM_CLASS (klass),
+                    G_SIGNAL_RUN_LAST,
+                    0,
+                    NULL, NULL, NULL,
+                    G_TYPE_NONE, 0);
 
   props[PROP_SESSION_MODE] =
     g_param_spec_string ("session-mode",
@@ -1872,3 +1880,9 @@ _shell_global_locate_pointer (ShellGlobal *global)
 {
   g_signal_emit (global, shell_global_signals[LOCATE_POINTER], 0);
 }
+
+void
+_shell_global_notify_shutdown (ShellGlobal *global)
+{
+  g_signal_emit (global, shell_global_signals[SHUTDOWN], 0);
+}