If the Window Manager supports the _NET_WM_STATE_HIDDEN, we use it to use
the _NET_WM_STATE protocol when de-iconifying windows (iconification is
unchanged, via XIconifyWindow). Additionally, we no longer interpret all
UnmapNotify events for our window as the result of iconification.

(Based on patch by Tomas Frydrych <tf@linux.intel.com>)