Abstract out "backdrop core" from `BackdropScaffold`

[daadu]

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!).