[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.
Good to know. Unfortunately at the moment it doesn't seem to make a difference anyway... 

Yes, I had set `layout.css.will-change.enabled` to True on desktop, but I didn't have much performance problems on desktop to begin with, I'm more interested in mobile - It's supposed to be enabled for privileged apps on Firefox OS, but maybe that was only for certified ? I need to double-check, that might be the issue...