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 introduces a new state of the backend called "setup". If the
music library path is not set, the backend goes into that state and the
app widget can show the UI to set it up. This also introduces a new
dependency on shared_preferences.
