Show radio stations in browser or as playlist

Depending on preferences of the user, we show radio stations as browsable directories or as playlist.

Browsable directories:
* Automatic reloading on every browse request
* Not all clients support it

Playlists:
* All clients support playlists
* Slow on initial load
* Radio stations are only reloaded on mopidy startup

Default configuration enables browsable directories and disables playlists.