[web-animations-1] Restructure and Improve DFNs in Animation Effects section - #8985

This makes it easier to understand what this object is
and to contextualize all the definitions that follow.

This improves the flow of the story we're telling
about what an animation effect is and how it works.

(See next commit for tweaks on the text; this only moves it.)

* give local time a proper DFN
* tweak Time Spaces to fit as an overview for more than just the iteration section
* use proper dfn markup for the various terms defined in the Time Spaces overview

…vs Progress Calculations

This restructuring sorts various subsections into:
* properties of an animation effect
   -> filed as direct subsections of Animation Effect
* transformation of the input time into output progress
   -> filed as direct subsections of Calculating Progress

This way the first section builds up the model of what an Animation Effect *is*,
and the second section defines the flow of time through various transformations into an output progress value.

(See next commit for integration edits; this only moves text.)

…timing function

* Integrate normative and non-normative text of “direction control”
  and give each playback direction value a proper definition.
* DFN a term for the effect-wide easing function, and clarify its effect.
* DFN “keyframe-specific timing function” for parallelism and easier referencing.

w3c#8985 (comment)