Add infrastructure to work out the best target given a pane or window

alone and use it to add pane_died and pane_exited hooks.

server.c

@@ -387,7 +387,7 @@ server_child_exited(pid_t pid, int status)
 		TAILQ_FOREACH(wp, &w->panes, entry) {
 			if (wp->pid == pid) {
 				wp->status = status;
-				server_destroy_pane(wp);
+				server_destroy_pane(wp, 1);
 				break;
 			}
 		}