Add a very simple GtkWidget function for an "tick" callback, which
is connected to the ::update signal of GdkFrameClock.

Remove:

 - GtkTimeline. The consensus is that it is too complex.
 - GdkPaintClockTarget. In the rare cases where tick callbacks
    aren't sufficient, it's possible to track the
    paint clock with ::realize/::unrealize/::hierarchy-changed.

GtkTimeline is kept using ::update directly to allow using a GtkTimeline
with a paint clock but no widget.