Try to read cover art from file in music_dir

Hey there,

In issue #97, two options for getting cover art are mentioned. This PR introduces the capability to prefetch cover art from the original file when it's present (both the file and the cover art therein).

It uses the code already implemented to fetch cover art from local files. To this end, this code is slightly tweaked.

There are still a few things todo:

import statements: for some reason I can't get them outside the class.
introduce settings for a download limit, and to enable the downloading of remote file
code cleanup

I made this a draft PR, no idea whether that's appropriate..

eonpatapon / mpDris2

Try to read cover art from file in music_dir #131