Add notify hooks for various events, the functions are currently empty

stubs but will be filled in for control mode later. From George Nachman.

cmd-new-window.c

@@ -89,6 +89,7 @@ cmd_new_window_exec(struct cmd *self, struct cmd_ctx *ctx)
 		 * Can't use session_detach as it will destroy session if this
 		 * makes it empty.
 		 */
+		notify_window_unlinked(s, wl->window);
 		wl->flags &= ~WINLINK_ALERTFLAGS;
 		winlink_stack_remove(&s->lastw, wl);
 		winlink_remove(&s->windows, wl);