GSConnect receives a lot of duplicate notifications, whether due to
reconnects, proactively requesting updates, etc.

Since GNOME 46, each of those repeats has been popped up as a new
banner notification every time it's received, even if nothing has
changed in the notification content. This is extremely disruptive.

To avoid that, don't blindly flag all repeated notifications as
"unacknowledged", which will cause them to be popped up again.
Only do so if the content has changed. (This covers re-displaying
banners for SMS conversations that receive an additional message,
since those are sent as updates to the existing notification.)

Fixes GSConnect#1855