Add window_activity format, from Thomas Adam based on a diff originally

from propos6 at gmail dot com.

tmux.h

@@ -892,6 +892,7 @@ struct window {
 	char		*name;
 	struct event	 name_timer;
 	struct timeval	 silence_timer;
+	struct timeval	 activity_time;
 
 	struct window_pane *active;
 	struct window_pane *last;