osc: kitty notifications: implement the 'close' request

Add a new config option, desktop-notifications.close, defining what to
execute to close a notification. It has a single template parameter,
${id}, that is expanded to the external notification ID foot may have
picked up from the notification helper.

notify-send does not support closing notifications, and it appears
impossible to pass an *unsigned* integer as argument to gdbus. Hence
no default value for the new 'close' option.

Example:

    printf '\e]99;i=123;this is a notification\e\\'
    printf '\e]99;i=123:p=close;\e\\'

config.h

@@ -340,6 +340,7 @@ struct config {
 
     struct {
         struct config_spawn_template command;
+        struct config_spawn_template close;
         bool inhibit_when_focused;
     } desktop_notifications;