Work out mouse scroll wheel effect when the mouse is first detected and

store it in struct mouse_event, reduce the scroll size the 3 but allow
shift to reduce it to 1 and meta and ctrl to multiply by 3 if the
terminal supports them, also support wheel in choose mode. From Marcel
Partap.

tmux.h

@@ -1168,6 +1168,7 @@ struct mouse_event {
 
 	u_int	button;
 	u_int	clicks;
+	u_int	scroll;
 
 	int	wheel;
 	int     event;