[web-animations-1][editorial] Revise animation dfn / intro

* Don't state the child relationship of animations to timelines
  as its informative definition; this is really confusing. It's
  not what an animation *is*, it's a side effect of the hierarchy.

* Move the nice, readable, understandable definition of what an
  animation effect is to the definition of animation effect,
  where it's desperately needed!

* Merge the remaining non-normative and normative definitions,
  which are half-duplicating each other already.

* Pull up the definition of “current time”, because it's quite
  fundamental here.

* Add a note about timeline vs animation notions of current time.

* Split up the definition of animation into logical paragraphs:
  1. binding effect and timeline (most fundamental)
  2. Role in time value flow
  3. Playback control features to manipulate said flow

* Reorganize the remaining paragraphs into a DL.

* Dfn-tag “seek” and cross-link it throughout

Author: fantasai