feat(neon_framework): use adaptive icons

[Leptopoda]

Just a small step towards a proper iOS design.
There are places where this change looks really good (e.g. the files app) and other ones where it currently looks a bit out of place (the notes bottom nav bar now is material design with cupertino icons).
I hope we can still land it :)

[provokateurin]

Should we make a script that prevents us using Icons. and MdiIcons. directly? I'm sure we and others will forget it otherwise

[Leptopoda]

I also thought about it but it might be more complex.

1. what about MdiIcons?
2. some icons do not have an apple equivalent (double checkmark to clear all notifications)
3. some icons are only ever shown on material platforms (the file picker dialog on iOS does not show the camera, folder and other icons). In general iOS dialogs do not use icons.

I think there would be a lot of false positives

[provokateurin]

Ok, but then we have to be careful when adding new icons anywhere.

[Leptopoda]

For now missing one should not be an issue as we don't yet take the iOS design seriously.
But once we support it we also have to be careful with the entire UI (thinking of #1626).

[provokateurin]

LGTM, but what about neon_talk?

[Leptopoda]

Just forgot it.
This is a broader question but should we add all the app specific icons to the framework or just have a few common ones with each app defining their own set (similar to what we do with the dialogs)?

[provokateurin]

Some additional icons won't hurt much and it will make it much easier for developers, so I'd say we should put the icons into the framework whenever possible.