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).

include/handlers.h

@@ -160,6 +160,13 @@ int handle_normal_hints(void *data, xcb_connection_t *conn, uint8_t state,
                         xcb_window_t window, xcb_atom_t name,
                         xcb_get_property_reply_t *reply);
 
+/**
+ * Handles the WM_HINTS property for extracting the urgency state of the window.
+ *
+ */
+int handle_hints(void *data, xcb_connection_t *conn, uint8_t state, xcb_window_t window,
+                  xcb_atom_t name, xcb_get_property_reply_t *reply);
+
 /**
  * Handles the transient for hints set by a window, signalizing that this
  * window is a popup window for some other window.