Feat: UserExceptionDialog using ScaffoldMessengaer

[SunlightBro]

UserExceptionDialog was written before flutter had Navigator 2.0 and the new scaffold-messenger

At the moment it still requires to place a UserExceptionDialog<St> in between the MaterialApp and the HomePage Widget, which makes it hard to work with MaterialApp.router.

If possible the best case would be, to not require a UserExceptionDialog Widget at all and instead directly use the ScaffoldMessenger to display the UserException with a SnackBar. But this would be a breaking change, so let me know if this is something you want.