Support iOS 10+ rich notifications (title, grouping)

[fridtjof]

Since iOS 10, notifications can have a title, making them easier to read by using the title to display, e.g. sender information.
Since iOS 12, notifications can be grouped, also improving the user experience a lot.

A good overview can be seen here (specifically about iOS 12 notifications):
https://developer.apple.com/videos/play/wwdc2018/710/

As this is a feature that I want very badly myself, I've been doing some work necessary to support this, however I can't test my changes because I'd need a paid developer account to do that (push entitlements etc). I will reference this issue in any PRs I make for this.

[fridtjof]

done via #2207, #2347