Add an UPDATE phase and GdkFrameClockTarget, use for GtkStyleContext

Switch GtkStyleContext to using GdkFrameClock. To do this, add a new
UPDATE phase to GdkFrameClock.

Add a GdkFrameClockTarget interface with a single set_clock() method,
and use this to deal with the fact that GtkWidget only has a frame
clock when realized.

https://bugzilla.gnome.org/show_bug.cgi?id=685460

gtk/gtkwidget.h

@@ -905,6 +905,13 @@ void                    gtk_widget_insert_action_group                  (GtkWidg
                                                                          const gchar  *name,
                                                                          GActionGroup *group);
 
+GDK_AVAILABLE_IN_3_2
+void gtk_widget_add_frame_clock_target (GtkWidget           *widget,
+                                        GdkFrameClockTarget *target);
+GDK_AVAILABLE_IN_3_2
+void gtk_widget_remove_frame_clock_target (GtkWidget           *widget,
+                                           GdkFrameClockTarget *target);
+
 G_END_DECLS
 
 #endif /* __GTK_WIDGET_H__ */