Add support for modifying a song's metadata

[SkyLeite]

Currently the Web Client only supports uploading album art. It would be nice if we could also change the song's metadata. I see that's already documented in the protocol section, so it makes me wonder why it's not implemented in the library.

[simon-weber]

The short answer is that developing against those apis was painful and not easily tested, so they're not maintained anymore. #490 added support for a few fields, though it's likely broken again by now.

[trjh]

I am presuming that this also holds true for Mobileclient.change_song_metadata() not being able to change the song['AlbumArtRef'][0]['url']. I think I saw on another API that it seems that Google itself is only allowing that method to change rating now.

It looks like it's been broken on the web UI for awhile https://productforums.google.com/forum/#!topic/play/nK9aTnusHh4

[SkyLeite]

That's not exactly related. As far as I can tell, the current problem when uploading album art is related to Google's american servers, since some people reported that it works under an European VPN. As for myself, I can confirm that uploading art on Firefox worked before this new issue appeared, so the one reported in that URL seems to have been fixed.