Group notifications using something other than the app (to support browsers)

[1ace]

Is your feature request related to a problem? Please describe.

Half-way between a bug report and a feature request.

For my use case, grouping by app is also useless, because firefox is the only app I use that sends notifications, so all the notifications from my personal & professional webmails and the three chat systems I have to use on various projects I work on (irccloud, element, discord... I wish I was joking) all get mixed together.

Describe the solution you'd like

Ideally there would be a better way to group notifications than by app, so that this feature can become useful (and it really could be useful!)

I'm not quite sure what other meta-data is available to group by; perhaps grouping by icon could work?

Describe alternatives you've considered

The request in #376 (to be able to disable this feature) would remove the problem, but I believe it's also possible to make grouping work.