Replace "Animatable" with "Animation type"

[birtles]

As discussed: https://lists.w3.org/Archives/Public/www-style/2015Oct/0226.html (item 8)

This will probably involve editing all the CSS specs that use this line and updating terms in CSS transitions too.

[birtles]

Need to be careful to mark the 'display' attribute as having 'Animation type: none' when doing this. See Mozilla bug 1264396 for some background.

[xfq]

Related: #3198

[birtles]

As pointed out by @xfq this has been done in #3198 and others.