change BoxScrollView to Widget (rebase of #12)

[thielepaul]

I rebased #12 on the current master as I wanted both the changes from #12 as well as null safety

To use the fork, one can add the following to the pubspec.yaml:

  draggable_scrollbar:
    git:
      url: https://github.com/thielepaul/flutter-draggable-scrollbar.git
      ref: cfd570035bf393de541d32e9b28808b5d7e602df