Playlist Server-side Preparations

[hasezoey]

This PR is a collection of changes in preparation for doing #152 cleanly (to be better reviewable). In more details:

• rename playback.rs to music_service_client.rs to be consistent with the server and be better find-able
• remove serde from termusic-playback as it has never been necessary there
• add a TuiCmd enum to use for tui command instead of sharing PlayerCmd (this is later necessary as both may need to carry different data, also now we can exhaustively match in the tui)
• dont cycle the playlist Loop-Mode in the tui, return it from the server on request and use that instead
• remove some unused variants from PlayerCmd
• remove Default impl for Playlist as it basically is not used and new should be used instead and match / error on the result
• add a new function to the podcast db to get a episode by it URI / URL
• some other general style changes