callback library Null safety

Since JSON does not support functions, we need to use a custom class to map intents to callbacks.

These classes do not depend on Flutter and can be extended to work with whichever runtime you are using.

Classes

BoolSelectionCallback
Callback
ClearMaterialBannersCallback
CopyClipboardCallback
EmptyCallback
HideCurrentMaterialBannerCallback
HideCurrentSnackBarCallback
HttpRequestCallback
IntSelectionCallback
ReloadCallback
RemoveCurrentMaterialBannerCallback
RemoveCurrentSnackBarCallback
ShowMaterialBannerCallback
ShowSnackBarCallback
UrlCallback