Boundary constants

[renancaraujo]

As asked by @sgon00, this PR adds the possibility of applying the states "covered" and "contained" as ´minScale´ or ´maxScale´.

new PhotoView(
  imageProvider: imageProvider,
  loadingChild: new LoadingText(),
  backgroundColor: Colors.white,
  minScale: PhotoViewScaleBoundary.contained,
  maxScale: PhotoViewScaleBoundary.covered
);