Fallback online sources

[mjholub]

Currently the app:

1. won't print a message when a track is not found in preferred source, unless launched from CLI
2. does not look for a track elsewhere if it doesn't find the song in preferred source. Some sources have their advantages, such as better quality, but may lack certain songs. It would be nice if the app either allowed users to set a secondary (fallback) source or looped over all available sources. It would be also appreciated to see the icon of the playback source in such arrangement.

[nukeop]

Some source is always found, we just rank them according to accuracy (in most cases it's just the first search result, but in the case of Youtube, we use some heuristics). At this stage we can't tell if the stream is going to fail (due to geoblocking, etc). If it does when it starts loading, a new stream should be found then.