Use maximum available resolution for thumbnail

[ritiek]

This PR improves thumbnail quality in metadata when using mpris API (instead of always falling back to default.jpg). One drawback is that this makes things a little slower (checking if URLs responds with 200 or not). Not sure if it is worth it though, let's decide! See #835 (comment).

Cross reference: mps-youtube/pafy#211

[ids1024]

Perhaps change this to call the method from pafy? That means it pulls in pafy's develop branch as a dependency (and we'll need to release a new pafy before/when releasing a new mps-youtube), but I think this is generally a reasonable thing to do.

[ritiek]

Ok, I've changed it to use Pafy.getbestthumb(). Since most people have youtube-dl installed when using mps-youtube, so there isn't going be any additional delay to fetch the best thumbnail mps-youtube/pafy#211 (comment).

Also, I think we should release a new version of pafy before merging this otherwise we'll get half the issues complaining about Pafy.getbestthumb method not found :D

[vn-ki]

Let's just remove the pafy internal backend. I don't think anyone uses it anymore.

[ritiek]

Actually, it still lags (about 3s) probably because of util.get_pafy. I'll have to see more into this.