model: Handle keymap variants

We supposedly handle the same "layout+variant" argument format than the old gkbd-keyboard-display (e.g. "fr+azerty"), but currently don't perform this separation in order to lookup the correct xkb_keymap.

Separate these two arguments, and fix the XKB keymap lookup to consider the variant being specified. Fixes invocation from Settings with any of the affected keymap options.

Closes: #7 (closed)