Replaces decoder_initialized().

Prepare for a Decoder API redesign based on an abstract class with
virtual methods.

If the duration is unknown, pass SignedSongTime::Negative(), as
documented for decoder_initialized().

This is obsolete because it has been moved to the MPD core.

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

Allow reusing Tag instances.

Remove GLib dependency from some headers.

Invoke decoder_initialized() in the libFLAC metadata callback.  This
merges code from the FLAC and the OggFLAC decoder plugin into the
common library.