Add an activity time for clients, like for sessions, and change session and

client lookup to pick the most recently used rather than the most recently
created - this is much more useful when used interactively and (because the
activity time is set at creation) should have no effect on source-file.

Based on a problem reported by Jan Johansson.

tmux.h

@@ -1064,6 +1064,7 @@ struct client {
 	struct imsgbuf	 ibuf;
 
 	struct timeval	 creation_time;
+	struct timeval	 activity_time;
 
 	struct environ	 environ;