[RFC] Subtitle cycle

This is same as #4030

* Added a CycleSubtitle action which cycles subtitles without toggling from disable/enable
* Fixed comment in key.h +cosmetics +spelling
* Squashed + rebased on master

As per @topfs2 comment he didn't have time and forgot about it, inviting someone to cherry-pick and squash + took opportunity to fix up spelling in comment.