Drop support for misencoded tags.  People should not be using ID3v1
anyway.

The new tag is supported by all decoders that use the tag name table,
and the ID3v2 tag "TSOA" maps to it.

The Release Track Id uniquely identifies a recording on a release - that
is, even if a recording appears twice on a release (meaning that the
combination of recording and release id are not enough to figure out
which one it is), the release track id will allow differentiating the two.

The tag names are taken from
https://musicbrainz.org/doc/MusicBrainz_Picard/Tags/Mapping

Replaces g_strstrip().

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

Reduce heap allocator overhead.

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

Replaces GLib's GError.

g_file_error_quark() is meant to be used with the GFileError enum
which does not correspond with errno, but must be converted with
g_file_error_from_errno().

At the same time, this removes g_strerror() use for
g_file_error_quark().

Make sure we get a UTF-8 encoded string.

Use _scan() instead, to have more control.

The new method pair() receives an arbitrary name/value pair.  Support
for this is being added to a few decoder plugins.

Pass a callback table to scan_file() and scan_stream(), instead of
returning a tag object.