actor: Simplify setters of animatable properties

Instead of checking the duration of the current easing state we should
check if there's a transition in progress, and update it
unconditionally.

If there is no easing state, or the easing state has a duration of zero
milliseconds, then create_transition() should bail out early and set the
requested final state.

This allows us to write:

  clutter_actor_save_easing_state (actor);
  clutter_actor_set_x (actor, 200);
  clutter_actor_restore_easing_state (actor);

  [...]

  clutter_actor_set_x (actor, 100);

and have the second set_x() update the easing in progress, instead of
being ignored.

https://bugzilla.gnome.org/show_bug.cgi?id=672945