[Xcb] Add xcb_size_hints_is_none() in xcb-util/icccm

[Barton C Massey]

In message <sa5bq4yg99x.fsf at Orfeo.duckcorp.org> you wrote:
> The second would  probably means to change a lot the  API (I don't think
> this is a  problem at the moment though...), because in  order to have a
> consistent API, if we remove xcb_size_hints_is_* functions, we also have
> to remove  xcb_wm_hints_state_is_* and xcb_wm_hints_is_*  functions, no?

Sounds good to me.

    Bart