renaming "Animatable:" to "Interpolation:" in property definition tables

In [1] and [2] the working group resolved that both animations and
transitions should work on properties that cannot be interpolated,
and these properties should switch when the timing function crosses
50%.  (This is particularly useful with the step-start and step-end
timing functions.)

Given this resolution, I think it no longer makes sense for the line
in the property definition table to be "Animatable:", as I suggested
on IRC in [1].

I'd like to change "Animatable:" to "Interpolation:", and change the
common "no" value to "discrete" or "in steps" or something similar.

This is basically an editorial change that follows from the existing
resolutions in [1] and [2], but since it affects multiple
specifications I wanted to give people a chance to comment.

-David

[1] https://lists.w3.org/Archives/Public/www-style/2012Nov/0261.html
[2] https://lists.w3.org/Archives/Public/www-style/2014Oct/0290.html

-- 
𝄞   L. David Baron                         http://dbaron.org/   𝄂
𝄢   Mozilla                          https://www.mozilla.org/   𝄂
             Before I built a wall I'd ask to know
             What I was walling in or walling out,
             And to whom I was like to give offense.
               - Robert Frost, Mending Wall (1914)