And drop some more guesswork in the code, since some layouts have
less than 4 levels. This also allows for having OSK maps with more
than 4 levels. Let us hope that the sanity of our future kin will
remain below that threshold.
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/3162>
Drop some code, in favor of a numeric keypad that is driven through
the same JSON-based maps. This is also a first use of the "height"
key property in the JSON files, for the Enter key.
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/3162>