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

Replaces GLib's GError.

Allow reusing Tag instances.

Let the function decoder_replay_gain() update
decoder_control::replay_gain_db instead of letting each decoder plugin
take care for that.

This has been deprecated by the "embcue" playlist plugin.

Adds mixrampdb and mixrampdelay commands.  Reads MIXRAP_START and
MIXRAMP_END tags from FLAC files and overlaps instead of crossfading.

Don't allocate each replay_gain_info object on the heap.  Those
objects who held a pointer now store a full replay_gain_info object.
This reduces the number of allocations and heap fragmentation.

Easier to reuse the function.

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.