servo: Merge #18604 - style: Cleanup the animated value setup (from emilio:animated-value-cleanup); r=nox

We have three different enums to represent slightly different things. Reuse them
properly, and kill some code in the animated_properties module while at it.

Source-Repo: https://github.com/servo/servo
Source-Revision: 9ffd2b8c9e2b897f12daa5eac48a76a0c10c60c9

--HG--
extra : subtree_source : https%3A//hg.mozilla.org/projects/converted-servo-linear
extra : subtree_revision : dd97b322e75cdcdf26f991e11da85056aad56cc6