Add new state API

Add new state API

Add new API to deal with xkb_state objects, including
xkb_state_update_key, which runs the XKB action machinery internally to
calculate what exactly happens to the state when a given key is pressed
or released.

The canonical way to deal with keys is now:
    struct xkb_state *state = xkb_state_new(xkb);
    xkb_keysym_t *syms;
    int num_syms;

    xkb_state_update_key(state, key, is_down);
    num_syms = xkb_key_get_syms(state, key, &syms);

More state handling API, including a way to get at or ignore preserved
modifiers, is on its way.

Signed-off-by: Daniel Stone <daniel@fooishbar.org>