libbuxton: don't change the key type

The key type is not used by any of the wiring
functions get_label, set_label, create_group
and remove_group. Then it is much better to not
alter the type of the key.

Signed-off-by: José Bollo <jose.bollo@open.eurogiciel.org>
libbuxton: enforcing correct type for groups

When creating group keys, it enforces the type to
be either STRING or UNSET.

Signed-off-by: José Bollo <jose.bollo@open.eurogiciel.org>