xkb: fix group wrapping

xkb: fix group wrapping

- Use the number of groups we actually have instead of the maximum
  number of groups, when wrapping according to group_wrap control.

- Be careful with negative relative group actions. The group fields of
  xkb_state are all unsigned (and have a weird FIXME..), so don't use
  them directly.

- Add the grp:ctrl_shift_toggle option to the rmlvo set to also test
  locking the previous group (i.e. a negative relative group action).

Signed-off-by: Ran Benita <ran234@gmail.com>
Signed-off-by: David Herrmann <dh.herrmann@googlemail.com>