Rework ElementAnimationSet::start_transition_if_applicable to be more like the specification (#145)

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

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

Author: yezhizhen