Add support for _NET_WM_STATE_DEMANDS_ATTENTION.

_NET_WM_STATE_DEMANDS_ATTENTION indicates that some action in or with
the window happened. It's a weaker hint than urgency flag of WM_HINTS,
but some applications and almost all Qt applications use it instead of
WM_HINTS' urgency flag (one example is Skype).

include/con.h

@@ -324,6 +324,12 @@ bool con_has_urgent_child(Con *con);
  */
 void con_update_parents_urgency(Con *con);
 
+/**
+ * Set urgency flag to the container, all the parent containers and the workspace.
+ *
+ */
+void con_set_urgency(Con *con, bool urgent);
+
 /**
  * Create a string representing the subtree under con.
  *