Don't re-show repeated notifications

[ferdnyc]

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 #1855

[andyholmes]

This seems reasonable, but I'm not familiar with the implementation anymore. If it works in practice, I'd say it's fine 👍

[ferdnyc]

If it works in practice

Yeah, that's the $64,000 question. I'm hoping it'll score at least one or two testers from #1855 to confirm.

Most I can say is, "it works for me"... but I don't feel like that represents nearly a wide enough range of scenarios. The apps I have notifications enabled for number fewer than a dozen — just enough for their constant re-displaying to annoy me into fixing it.

My limited local testing did include a Google Messages notification, which no longer endlessly repeated, but still popped up anew when a new SMS came in that was added to that conversation. But "saw it work exactly once" feels more like a reason not to halt further testing, than to declare unqualified success.