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

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

Replaces GLib's GError.

Allow reusing Tag instances.

Avoid duplicating the tag.

Implement the "scan_stream" method that can read tags from any
input_stream object.  This requires a FLAC__IOCallbacks implementation
based on the input_stream API.

Merge common code.

Not using libFLAC++ because this library adds a lot of overhead due to
virtual method calls.  This new class library is zero-overhead.

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

Allow the function to be cancelled.

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