EpisodeSource and ShowSource is too hard-coded

[tobinus]

Ideally, it should be easy to switch from one implementation of EpisodeSource to another. For example, instead of using Radio Rest API we could have a source which uses a file system, configuration files, (local) database, SoundCloud and so on. Currently, doing so requires an extensive amount of work and duplication, since EpisodeSource implements both specific methods applicable for only this implementation and more general methods applicable to all implementations.

EpisodeSource and ShowSource should follow the Metadata Sources, in that there are abstract base classes available which concrete implementations can use. I suggest we still hold on to the limitation of having only one EpisodeSource and ShowSource active at any one time, but it might also be possible to associate different EpisodeSources with different shows. That last bit would be better saved for another time, though. If multiple sources are to be used, then an implementation could be created which merges two (or more) other sources.