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This is obsolete because it has been moved to the MPD core.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

This feature has been moved to the "flac" playlist plugin.

This function replaces the replay_gain_info parameter for
decoder_data().  This allows the decoder to announce replay gain
changes, instead of having to pass the same object over and over.

This patch prepares support for floating point samples (and probably
other formats).  It changes the meaning of the "bits" attribute from a
bit count to a symbolic value.