Abstract out "backdrop core" from `BackdropScaffold`

Something like `BackdropController` or `BackdropLayout` or just `Backdrop` - that has all the `backdrop` logic/animation in it. This will make the functionality re-usable with other non-scaffold like scenario - like `NavigationRails` #17, with Horizontal layout #75 etc. This issue could be pre-requisite for it.

This will also increase the usability of the package in-general.

This is in-line with what @willlarche [suggested when I posted about the package in flutter-dev group (wayback!)](https://groups.google.com/g/flutter-dev/c/NSbB6KhjgKc/m/jJfdEawFCQAJ).