Add a framework for restarting the compositor with nice visuals

The current GNOME Shell Alt-F2 restart looks very messy and also
provides no indication to the user what is going on. We need to
restart the compositor to switch in and out of stereo mode, so
add a framework for doing this more cleanly:

Additions:

 meta_restart(): restarts the compositor with a message
 MetaDisplay::show-restart-message: signal the embedding
    shell to show a message
 MetaDisplay::restart: signal the embedding shell to restart
    itself.
 meta_is_restart(): indicates whether the current instance is a
                    restart so we can suppress login animations.

A helper program meta-restart-helper holds the composite overlay
window up during the restart to avoid visual artifacts.

src/core/display.c

@@ -130,6 +130,8 @@ enum
   WINDOW_MARKED_URGENT,
   GRAB_OP_BEGIN,
   GRAB_OP_END,
+  SHOW_RESTART_MESSAGE,
+  RESTART,
   LAST_SIGNAL
 };
 
@@ -306,6 +308,59 @@ meta_display_class_init (MetaDisplayClass *klass)
                   META_TYPE_WINDOW,
                   META_TYPE_GRAB_OP);
 
+  /**
+   * MetaDisplay::show-restart-message:
+   * @display: the #MetaDisplay instance
+   * @message: (allow-none): The message to display, or %NULL
+   *  to clear a previous restart message.
+   *
+   * The ::show-restart-message signal will be emitted to indicate
+   * that the compositor should show a message during restart. This is
+   * emitted when meta_restart() is called, either by Mutter
+   * internally or by the embedding compositor.  The message should be
+   * immediately added to the Clutter stage in its final form -
+   * ::restart will be emitted to exit the application and leave the
+   * stage contents frozen as soon as the the stage is painted again.
+   *
+   * On case of failure to restart, this signal will be emitted again
+   * with %NULL for @message.
+   *
+   * Returns: %TRUE means the message was added to the stage; %FALSE
+   *   indicates that the compositor did not show the message.
+   */
+  display_signals[SHOW_RESTART_MESSAGE] =
+    g_signal_new ("show-restart-message",
+                  G_TYPE_FROM_CLASS (klass),
+                  G_SIGNAL_RUN_LAST,
+                  0,
+                  g_signal_accumulator_true_handled,
+                  NULL, NULL,
+                  G_TYPE_BOOLEAN, 1,
+                  G_TYPE_STRING);
+
+  /**
+   * MetaDisplay::restart:
+   * @display: the #MetaDisplay instance
+   *
+   * The ::restart signal is emitted to indicate that compositor
+   * should reexec the process. This is
+   * emitted when meta_restart() is called, either by Mutter
+   * internally or by the embedding compositor. See also
+   * ::show-restart-message.
+   *
+   * Returns: %FALSE to indicate that the compositor could not
+   *  be restarted. When the compositor is restarted, the signal
+   *  should not return.
+   */
+  display_signals[RESTART] =
+    g_signal_new ("restart",
+                  G_TYPE_FROM_CLASS (klass),
+                  G_SIGNAL_RUN_LAST,
+                  0,
+                  g_signal_accumulator_true_handled,
+                  NULL, NULL,
+                  G_TYPE_BOOLEAN, 0);
+
   g_object_class_install_property (object_class,
                                    PROP_FOCUS_WINDOW,
                                    g_param_spec_object ("focus-window",
@@ -5939,3 +5994,28 @@ meta_display_clear_mouse_mode (MetaDisplay *display)
 {
   display->mouse_mode = FALSE;
 }
+
+gboolean
+meta_display_show_restart_message (MetaDisplay *display,
+                                   const char  *message)
+{
+  gboolean result = FALSE;
+
+  g_signal_emit (display,
+                 display_signals[SHOW_RESTART_MESSAGE], 0,
+                 message, &result);
+
+  return result;
+}
+
+gboolean
+meta_display_request_restart (MetaDisplay *display)
+{
+  gboolean result = FALSE;
+
+  g_signal_emit (display,
+                 display_signals[RESTART], 0,
+                 &result);
+
+  return result;
+}