Implement vim-like marks

Commands are 'mark' and 'goto'. Both can be used either directly,
like 'mark a' and 'goto a', or interactively (just 'mark'). For
interactive mode, i3-input must be installed and in your PATH.

include/client.h

@@ -97,6 +97,13 @@ void client_unmap(xcb_connection_t *conn, Client *client);
  */
 void client_map(xcb_connection_t *conn, Client *client);
 
+/**
+ * Set the given mark for this client. Used for jumping to the client
+ * afterwards (like m<mark> and '<mark> in vim).
+ *
+ */
+void client_mark(xcb_connection_t *conn, Client *client, const char *mark);
+
 /**
  * Pretty-prints the client’s information into the logfile.
  *