Fixed handling of UnknownFrames in ID3v2.

- If an unknown frame with id "XXXX" occurs, an entry
"UNKNOWN/XXXX" is added to unsupportedData().
The removeUnsupportedProperties() method in turn
removes all unknown frames with id "XXXX" if it
encounters a string "UNKNOWN/XXXX" in the given list.

- Implemented findByDescription() to UnsynchronizedLyricsFrame
in order to support removal of lyrics frames with unsupported
keys.

- Adapted id3v2 test case to new QuodLibet policy.