Add a ##timelineitems.isprivate## field in notification templates

[troncoli]

As mentionned in this forum post https://forum.glpi-project.org/viewtopic.php?id=294766, when iterating through the last n timeliteitems in a notification template, it would be interesting to be able to check if the timelineitems is private.

In my use case, I am reproducing the ticket feed in the notification, and being able to detect if the followup/task is private would allow me to put a dashed border like in GLPI for better readability.