Rework ElementAnimationSet::start_transition_if_applicable to be more like the specification

This improves the behavior of transition toggling, cancelation, and
delay. In particular there are many improvements with how transitions
are started when replacing an existing one.

Co-authored-by: Martin Robinson <[email protected]>
Signed-off-by: Euclid Ye <[email protected]>
Signed-off-by: Martin Robinson <[email protected]>

Author: yezhizhen