player: Revamp playlists and discovery

Here is an attempt to separate the Player logic from its Playlist. The whole repeat logic (in particular shuffle) should be in a much better state.

Some thoughts:

• it's really difficult to rewrite things with the current state of mpris
• songs are discovered on load. I'm not sure this is a good approach. Well, I'm not even sure this whole discovery thing is a good idea
• I'm afraid of misunderstandings between player playlist and playlists

Update june 19

• rename discovery to validation
• repeats: use only RepeatMode term
• fix a bug in songs validation if the same song is present multiple times
• fix typos

Update august 14

• add some missing docstrings in PlayerPlaylist and Player
• complete rewrite of the whole validation