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.
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.
