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

src/ewmh.c

@@ -71,6 +71,22 @@ void ewmh_update_workarea(void) {
     xcb_delete_property(conn, root, A__NET_WORKAREA);
 }
 
+/*
+ * Updates the _NET_CLIENT_LIST hint.
+ *
+ */
+void ewmh_update_client_list(xcb_window_t *list, int num_windows) {
+    xcb_change_property(
+        conn,
+        XCB_PROP_MODE_REPLACE,
+        root,
+        A__NET_CLIENT_LIST,
+        XCB_ATOM_WINDOW,
+        32,
+        num_windows,
+        list);
+}
+
 /*
  * Updates the _NET_CLIENT_LIST_STACKING hint.
  *
@@ -122,5 +138,5 @@ void ewmh_setup_hints(void) {
     /* I’m not entirely sure if we need to keep _NET_WM_NAME on root. */
     xcb_change_property(conn, XCB_PROP_MODE_REPLACE, root, A__NET_WM_NAME, A_UTF8_STRING, 8, strlen("i3"), "i3");
 
-    xcb_change_property(conn, XCB_PROP_MODE_REPLACE, root, A__NET_SUPPORTED, XCB_ATOM_ATOM, 32, 18, supported_atoms);
+    xcb_change_property(conn, XCB_PROP_MODE_REPLACE, root, A__NET_SUPPORTED, XCB_ATOM_ATOM, 32, 19, supported_atoms);
 }