Add ability to switch keyboard layouts (groups) on the fly

We sorta already have this, via the (poorly-named) "Change layout option" setting in the keyboard dialog, which sets /Default/XkbOptions/Group. But there may be more to do (e.g. sync to all keyboard devices if there's more than one).

It would also be good to have a visual indication of the group switching (sort of like a tabwin for layouts).

On top of that, we should be able to offer UI for to change the active group, but that would require a custom protocol or something.