Add semi-complete support for multiple artists.
This changeset completely reworks the music linker to add the following
new behaviors:
1. Artists are now derived from both artist and album artist tags,
with them being linked to songs and albums respectively
2. Albums and songs can now have multiple artists that can be distinct
from eachother
3. Previous Genre picking infrastructure has been removed and replaced
with artist picking infrastructure. "Play from genre" has been retired
entirely.
This is a clean break to the previous artist model and may not work
with all libraries. Steps to migrate the music library will be added
to the changelog.
Resolves#195.
62ee46cfe6