first try to fix fullscreen
Set _NET_WM_STATE when the client requests to do so (fixes fullscreen)

Tested with mplayer2, chrome and firefox.

Also refactored handle_ClientMessage to use sets to handle the state
flags, and added a small TODO to xcbq's get_net_wm_state(), pointing out
that it returns only the first item of the list (this doesn't affect
anything as long as we support only _NET_WM_STATE_FULLSCREEN)
Implement a few essential EWMH hints

Including _NET_SUPPORTED, _NET_SUPPORTING_WM_CHECK and _NET_WM_NAME.

Fixes fullscreen on gnome-terminal, which checks first if there's an
EWMH compliant window manager using _NET_SUPPORTING_WM_CHECK, then looks
into _NET_SUPPORTED for the needed flags (_NET_WM_STATE_FULLSCREEN)

(The xcbq.SUPPORTED_ATOMS list is obviously incomplete, please add more
if needed)
Honour auto_fullscreen config.

Check auto_fullscreen setting before analysing the _NET_WM_STATE
ClientMessage.