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\\'

foot.ini

@@ -48,6 +48,7 @@
 
 [desktop-notifications]
 # command=notify-send --wait --app-name ${app-id} --icon ${icon} --urgency ${urgency} --action ${action-name}=${action-label} --print-id -- ${title} ${body}
+# close=""
 # inhibit-when-focused=yes