Add ctrl:swapcaps_and_switch_layout

It's impossible to use grp:caps_toggle together with ctrl:swapcaps with libxkbcommon. I brought it up there, and the maintainer told me I could add my own option or bring it up in xkeyboard-config. Well, this is the new option.

If this is the right way forward (compared to re-ordering existing options or something, I haven't tried it because I'm not sure what could possibly break from such a change), then I'd like to request help with naming, not sure what would be a better name for this.