Hook GtkTimeline up to GdkFrameClock

Use GdkFrameClock for the timing of GtkTimeline. This require the
user to provide either a GtkWidget or a GdkFrameClock when creating
the timeline. The default constructor now takes a GtkWidget. If you
want to create a GdkFrameClock without a widget, you need to use
g_object_new() and pass in a GdkFrameClock and GdkScreen.

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

gtk/gtk.h

@@ -194,6 +194,7 @@
 #include <gtk/gtktexttagtable.h>
 #include <gtk/gtktextview.h>
 #include <gtk/gtkthemingengine.h>
+#include <gtk/gtktimeline.h>
 #include <gtk/gtktoggleaction.h>
 #include <gtk/gtktogglebutton.h>
 #include <gtk/gtktoggletoolbutton.h>