Obsolete class CloseSongEnumerator, which was a kludge.

This reverts commit d7d9dbd2 by
reimplementing it with the current MPD API.

3 years ago, I was wrong about the "embcue" plugin being able to
replace this one, because "embcue" reads a tag named "CUESHEET", while
this plugin reads the "CUESHEET" FLAC metablock.  There's an important
difference between those two!

The old struct config_param remains only for top-level string options.

Don't use GLib.

Simplify the definition of many build options.

Use the new uri_get_suffix() overload that removes the query string.

Simplifies a lot of code, because we don't need to return both the
SongEnumerator and the InputStream.

Merge duplicate code.

Merge some duplicate code.

Replaces g_uri_parse_scheme().

Decouple some more from GLib.

Prepare to migrate away from GLib.  Currently, we're still using GLib
as a backend.

This plugin has been defunct ever since Last.fm changed their
protocol.  Since there is no volunteer willing to fix the plugin, I'm
removing it now.

Migrate the remaining callers to FatalError().

Replaces struct playlist_provider.

Use only ConfigData.hxx in plugin sources to reduce header
dependencies.