Enables the use of a new `xkb-model` dconf key to let user configure
their desired keyboard model.
Also included is additional glue for respecting X11Model set at the
system level (ie. via `localectl`).
For simplicity, this assumes the use of one global keyboard model.
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2583>