Maintain the _NET_CLIENT_LIST property

Add and update the _NET_CLIENT_LIST property on the root window to
better comply with ewmh standards.

Information on this property can be found here:
http://standards.freedesktop.org/wm-spec/latest/ar01s03.html

> These arrays contain all X Windows managed by the Window Manager.
> _NET_CLIENT_LIST has initial mapping order, starting with the oldest window.

fixes #1099

include/ewmh.h

@@ -27,6 +27,11 @@ void ewmh_update_current_desktop(void);
  */
 void ewmh_update_active_window(xcb_window_t window);
 
+/**
+ * Updates the _NET_CLIENT_LIST hint. Used for window listers.
+ */
+void ewmh_update_client_list(xcb_window_t *list, int num_windows);
+
 /**
  * Updates the _NET_CLIENT_LIST_STACKING hint. Necessary to move tabs in
  * Chromium correctly.