Add flags to refresh-client (-U -D -L -R and -c) to pan a window that

is larger than the client manually. Bound to S-Up, S-Down, S-Left,
S-Right and Delete manually.

Also add aliases for keys DC = Delete, IC = Insert, and make
refresh-client -C accept XxY as well as X,Y to match default-size.

tmux.h

@@ -1421,6 +1421,10 @@ struct client {
 
 	int		 references;
 
+	void		*pan_window;
+	u_int		 pan_ox;
+	u_int		 pan_oy;
+
 	TAILQ_ENTRY(client) entry;
 };
 TAILQ_HEAD(clients, client);