Add the CD Cover of playing track for music modules

I already have started yesterday with the Rhythmbox module.

Here is the Rhythmbox module workflow:
  1: Request the cover file path to Rhythmbox through D-Bus
  2: If nothing return, search in the current song path for file named 'cover' and having file extension 'jpg', 'jpeg', 'gif', 'png', ...
  3: If nothing found, search in the Rhythmbox cache folder for file with the pattern 'artist - album' and having file extension 'jpg', 'jpeg', 'gif', 'png', ...
  4: If nothing found, and finally, try to download it from amazon and save it to the Rhythmbox cache folder if the enable_cover_download parameter is enabled

I hope for other music players it will be more easiest. ;)

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It's now implemented into Exaile module.