[WIP, don't merge] Improve throbber/spinner animations smoothness (bug 1091654)

https://bugzilla.mozilla.org/show_bug.cgi?id=1091654

* `.spinner` no longer contains the animation, it's inside a `::before` to avoid repainting more than necessary when there is padding added to the spinner
* `.throbber b` animation no longer changes the background, instead we change the opacity of another layer with the different background in each `<b>`

WIP, still need to test & improve with Firefox OS / Firefox Desktop devtools to figure out what we can do.
I filed https://bugzilla.mozilla.org/show_bug.cgi?id=1092144 btw to get `will-change` enabled in privileged apps.