[modules/layout-xkb] Use set-xkbmap to switch layouts
[modules/layout-xkb] Use set-xkbmap to switch layouts

Use the data retrieved via xkb and set the new layouts and variants
using set-xkbmap.

see #177

tobi-wan-kenobi/bumblebee-status,tobi-wan-kenobi/bumblebee-status