Implement the urgency hint for windows/workspaces

Thanks to Mikael for bringing it to my mind. This change introduces
two new color classes, client.urgent and bar.urgent. By default,
urgent clients are drawn in red (colors by Atsutane).

src/mainx.c

@@ -398,6 +398,9 @@ int main(int argc, char *argv[], char *env[]) {
         /* Watch WM_CLIENT_LEADER (= logical parent window for toolbars etc.) */
         xcb_property_set_handler(&prophs, atoms[WM_CLIENT_LEADER], UINT_MAX, handle_clientleader_change, NULL);
 
+        /* Watch WM_HINTS (contains the urgent property) */
+        xcb_property_set_handler(&prophs, WM_HINTS, UINT_MAX, handle_hints, NULL);
+
         /* Set up the atoms we support */
         check_error(conn, xcb_change_property_checked(conn, XCB_PROP_MODE_REPLACE, root, atoms[_NET_SUPPORTED],
                        ATOM, 32, 7, atoms), "Could not set _NET_SUPPORTED");