use xcb_window_t, add _NET_WM_PID too

 - _NET_WM_PID is not required by the standard but is often set
   by window managers anyway

 - the check window has no events like bspwm because the window
   is not reused by 2bwm for other things

 - bspwm only makes it the size of the desktop because it is also
   using it to capture motion events

 - bwpwm gets the order wrong, it is important to set up the
   properties on the check window _before_ setting the
   _NET_SUPPORTING_WM_CHECK on root.  Toolkits looking for the
   check window can fail if the root property is set before the
   window property.