Port to libmediaart

https://bugzilla.gnome.org/show_bug.cgi?id=709496
albumArtCache: Remove unused functions and constants

Cleanup for port to libmediaart.
build: libmediaart requirement is strict
travis: install libmediaart-dev
More fixes to Add to Playlist dialog

- Fix separators of the list in dark theme
- Add border to list
- Change background on hover