Ignore UnmapNotify if the window wants to enter WithdrawnState

Long description:

more info:
    http://tronche.com/gui/x/icccm/sec-4.html#s-4.1.4
    http://comments.gmane.org/gmane.comp.misc.suckless/6853

fixes #787

To apply this patch, use:
curl http://cr.i3wm.org/patch/142/raw.patch | git am

@@ -457,6 +457,13 @@ static void handle_unmap_notify_event(xcb_unmap_notify_event_t *event) {

         goto ignore_end;

     }

+    /* ignore UnmapNotify if the window wants to enter WithdrawnState

+     * more info: http://tronche.com/gui/x/icccm/sec-4.html#s-4.1.4

+     *            http://comments.gmane.org/gmane.comp.misc.suckless/6853

+     * this fix #787 */

+    if (event->from_configure == 0)

+        goto ignore_end;

+

     /* See the end of this function. */

     cookie = xcb_get_input_focus(conn);