Fix/play nav

[janbar]

Hi, here some changes to fix browse issues in the music library (plugin mpd). It was'nt a simple work and I had to cleanup to make it work properly.

• It fix broken links and play uri for many case including compilations.
• It fix album art: Using existing tag 'albumartist' or artist sort for items without the required tag.
• It fix search depending of artistsort