Add a simple keyboard layout switcher.

The keybind for it is Ctrl-Tab, which seems to be one of the few that:

- Don't emit a keycode usable in a password.
- Aren't selectable as Xkb modifier (as that would cause us to swich
  twice, which with 2 layouts would be a NOP).
- Are available on all layouts and keyboards I know.

Thanks to Soumya S. for an initial implementation of this, which I've
ported into auth_pam_x11!