notify: don't focus/report on notification dismissal

Only do it when the notification was activated.

Here, activated means the 'click to activate' notification action was
triggered.

How do we tie everything together?

First, we add a new template parameter, ${action}. It's intended to be
used with e.g. notify-send's --action option.

When the action is triggered, notify-send prints its name on stdout,
on a separate line. Look for this in stdout. Only if we've seen it do
we focus/report the notification.

notify.h

@@ -45,6 +45,7 @@ struct notification {
      * Used internally by notify
      */
 
+    bool activated;        /* User 'activated' the notification */
     char *xdg_token;       /* XDG activation token, from daemon */
 
     pid_t pid;             /* Notifier command PID */