[animation] Add Animation::update()

Currently, to update a property inside an animation you have to
get the interval for that property and then call the set_final_value()
method.

We can provide a simpler, bind()-like method for the convenience of
the developers that just validates everything and then calls the
Interval.set_final_value().

doc/reference/clutter/clutter-sections.txt

@@ -1436,6 +1436,7 @@ clutter_animation_completed
 <SUBSECTION>
 clutter_animation_bind
 clutter_animation_bind_interval
+clutter_animation_update
 clutter_animation_update_interval
 clutter_animation_has_property
 clutter_animation_unbind_property