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/workspace.c

@@ -351,3 +351,21 @@ void workspace_unmap_clients(xcb_connection_t *conn, Workspace *u_ws) {
         ignore_enter_notify_forall(conn, u_ws, false);
 }
 
+/*
+ * Goes through all clients on the given workspace and updates the workspace’s
+ * urgent flag accordingly.
+ *
+ */
+void workspace_update_urgent_flag(Workspace *ws) {
+        Client *current;
+
+        SLIST_FOREACH(current, &(ws->focus_stack), focus_clients) {
+                if (!current->urgent)
+                        continue;
+
+                ws->urgent = true;
+                return;
+        }
+
+        ws->urgent = false;
+}