Implement missing xkb_state_ref and add return value

Implement missing xkb_state_ref and add return value

xkb_state_ref was missing.

Also modify the _ref functions to return the object instead of being
void. This is a useful idiom:

    struct my_object my_object_new(struct xkb_state *state)
    {
        [...]
        my_object->state = xkb_state_ref(state);
        [...]
    }

Essentially "taking" a reference, such that you don't forget to
increment it and it's one line less (see example in our own code).

A case could also be made for _unref to return the object or NULL, but
this is quite uncommon.

Signed-off-by: Ran Benita <ran234@gmail.com>
[daniels: Updated for xkb_keymap changes.]