[Skin] Add CSS hook for action ("action-.." class on body)

[Skin] Add CSS hook for action ("action-.." class on body)
* Fixes bug 4438
* Depends on r108342. See also r108343 which did a similar thing for mw.config wgAction

Previously attempted in r91871, which was reverted in r94131.

Added in OutputPage instead of Skin::getPageClasses, as this is not directly page nor skin related. It also ensures independency from skins, this CSS class should be always available so that it can be relied on by the front-end.