Replaces GLib's GError.

Use the C++ API.

Optimize the ProxyDatabase by invoking "list" on the peer, instead of
visiting all songs.

Implements error checking for the gmtime_r() call, which was missing
in two code locations.

Let the database plugin do the match.

Merge locate_song_search() and locate_song_match().

Fold the case during construction, without having to create another
copy.

This finally enables the new embedded CUE sheet code: when a song file
contains a playlist, it is printed in the "lsinfo" output, so clients
get to know about this.

The protocol mandates that playlist URIs are absolute (i.e. full URI
relative to the music directory, not relative to the parent
directory).  This adds the parameter "directory" to the "playlist"
visitor method.

Use it in handle_lsinfo(), and eliminate some duplicate code.

Add nonnull attributes and fix all callers.

Use this struct for db_walk().