make _BackdropScaffoldState public and document it

[daadu]

• make _BackdropScaffoldState public
• members to be documented
  • AnimationController get controller
  • bool get isBackLayerConcealed
  • bool get isBackLayerRevealed
  • void fling()
  • void revealBackLayer()
  • void concealBackLayer()

[daadu]

I would suggest we also use the term "reveal" and "conceal" every where in documentation where ever we have mentioned "backLayer is visible" or "frontLayer is visible"

[daadu]

This should be worked on after #32 is merged with master