Add gdk_frame_clock_begin/end_updating()

Add an API to start or stop continually updating the frame clock.
This is a slight convenience for applcations and avoids the problem
of getting one more frame run after an animation stops, but the
primary motivation for this is because it looks like we might have
to use timeBeginPeriod()/timeEndPeriod() on Windows to get reasonably
accurate timing, and for that we'll need to know if there is an
animation running.

https://bugzilla.gnome.org/show_bug.cgi?id=693934

tests/video-timer.c

@@ -350,8 +350,6 @@ on_update (GdkFrameClock *frame_clock,
 
       gtk_widget_queue_draw (window);
     }
-
-  gdk_frame_clock_request_phase (frame_clock, GDK_FRAME_CLOCK_PHASE_UPDATE);
 }
 
 static GOptionEntry options[] = {
@@ -393,7 +391,7 @@ main(int argc, char **argv)
   frame_clock = gtk_widget_get_frame_clock (window);
   g_signal_connect (frame_clock, "update",
                     G_CALLBACK (on_update), NULL);
-  gdk_frame_clock_request_phase (frame_clock, GDK_FRAME_CLOCK_PHASE_UPDATE);
+  gdk_frame_clock_begin_updating (frame_clock);
 
   gtk_main ();