notify: close notification by sending SIGINT to helper

If the user hasn't configured a custom 'desktop-notifications.close'
command, try to close the notification by sending SIGINT to the
notification helper.

This is best-effort:

* If there's no helper running, we do nothing (except warn)
* We don't verify, in any way, the notification is actually closed
* We don't send any other signals, under any circumstances. That is,
  no SIGTERM, no SIGKILL. Ever.

osc.c

@@ -759,6 +759,7 @@ kitty_notification(struct terminal *term, char *string)
             .when = when,
             .urgency = urgency,
             .focus = focus,
+            .may_be_programatically_closed = true,
             .report_activated = report_activated,
             .report_closed = report_closed,
             .stdout_fd = -1,