Fix crash on select dupe song in playlist

[soncaokim]

Fixes #936

This is quite a big change, involving the introduction of unique and stable ID for each song in a static playlist (on the same idea that @Octoton implemented on the playing queue with the IndexedSong class).

To support this, I needed to change the API of multiple classes, from using ArrayList<Song> to List<? extends Song>.

I took the opportunity to do some cleanup and limited the concept of IndexedSong to only where it's necessary.

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Note that with this PR, if one select a duplicated item in a playlist, both copies will be selected. Any subsequent action on the selection (i.e. remove from playlist) will be applied to all the dupes (i.e. all occurence of the selected song will be removed).

Quite counter-intuitive for me.

Another PR (with important refactoring on the handling of multi-select in the case of dupe) will be proposed to handle this part. Relevant code are in https://github.com/VinylMusicPlayer/VinylMusicPlayer/tree/refactor-multi-select

[soncaokim]

@AdrienPoupa @Octoton Can you please have a look on this PR? I've updated the description to explain the reason behind this big change.

Thanks!