GdkFrameClock: Clean up the public API

* remove gdk_frame_clock_get_frame_time_val(); a convenience
  function that would rarely be used.
* remove gdk_frame_clock_get_requested() and
  ::frame-requested signal; while we might want to eventually
   be able to track the requested phases for a clock, we don't
  have a current use case.
* Make gdk_frame_clock_freeze/thaw() private: they are only
  used within GTK+ and have complex semantics.
* Remove gdk_frame_clock_get_last_complete(). Another convenience
  function that I don't have a current use case for.
* Rename:
  gdk_frame_clock_get_start() => gdk_frame_clock_get_history_start()
  gdk_frame_clocK_get_current_frame_timings() => gdk_frame_clock_get_timings()