Library search: fix wrong parentheses

Use lower() on match string, not on matching result.
MPDSong: provide values() method

Provide missing values() method for MPDSong by using the corresponding
method of the underlying mapping. This is used in library search over all
attributes.
packaging: increment version number in docs and code
Searching the library wasn't working at all.

Merge remote-tracking branch 'others/kurisuke/fix-library-search'