Everything related to file system access has been rewritten to make use
of the storage access framework. This means that the
WRITE_EXTERNAL_STORAGE is no longer needed. Because of that, the
dependency on permission_handler could be dropped and all code related
to permission handling has been removed. To be able to open a whole
document tree, the minSdkVersion was bumped to 21. Finally the file
selector was rewritten using custom platform dependent code.
This required to add a platform channel with a method to get the
toplevel storage devices. The app will need to request the
WRITE_EXTERNAL_STORAGE permission later. Also the dependency on "path"
was introduced.
