It's unlikely that anyone will want to have, say, a GtkWidget that
also acts as a GdkFrameClock, so an abstract base class is as
flexible as making GdkFrameClock an interface, but has advantages:

 - If we decide to never make implementing your own frame clock
   possible, we can remove the virtualization.
 - We can put functionality like history into the base class.
 - Avoids the oddity of a interface without a public interface
   VTable, which may cause problems for language bindings.