Add GdkFrameClock

Add an object GdkFrameClock that we associate with a GdkWindow.
This tracks when the window needs to be repainted, and will also
be used for other operations in the future like relayout and
updating animations.

Based on a patch from Havoc Pennington:

 https://mail.gnome.org/archives/gtk-devel-list/2010-October/msg00004.html

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

gtk/gtkwidget.h

@@ -473,6 +473,8 @@ void	   gtk_widget_queue_draw_region   (GtkWidget	       *widget,
                                            const cairo_region_t*region);
 void	   gtk_widget_queue_resize	  (GtkWidget	       *widget);
 void	   gtk_widget_queue_resize_no_redraw (GtkWidget *widget);
+GdkFrameClock* gtk_widget_get_frame_clock (GtkWidget           *widget);
+
 GDK_DEPRECATED_IN_3_0_FOR(gtk_widget_get_preferred_size)
 void       gtk_widget_size_request        (GtkWidget           *widget,
                                            GtkRequisition      *requisition);