Prefer tag data over directory names to display artist information

[timoreimann]

Supersonic uses directory names instead of tag information to represent artist labels. This is bad for at least the following reasons:

• Malformed or unsupported encodings can prevent directory information from being displayed properly.
• Information from audio tag fields properly set and "aligned" are not leveraged.
• Overall, the implied inconsistency makes Supersonic harder to understand, debug, and extend.

Therefore, artist labels should be derived from tag data. Since not every file can be expected to provide complete tag data, a fallback mechanism should still use directory names.

Side note: The long-term goal would probably be to consider tag data everywhere. However, for a starter the focus should be put on this issue.

This feature request affects issue #54 and #68.