Bugfix: Setup the _NET_SUPPORTING_WM_CHECK atom in a standards-compliant way

This makes chrome (and probably other apps, too) receive changes of the
_NET_ACTIVE_WINDOW and thus show its bookmark dialog properly.

Fixes: #544

include/ewmh.h

@@ -40,4 +40,10 @@ void ewmh_update_active_window(xcb_window_t window);
  */
 void ewmh_update_client_list_stacking(xcb_window_t *stack, int num_windows);
 
+/**
+ * Set up the EWMH hints on the root window.
+ *
+ */
+void ewmh_setup_hints();
+
 #endif