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.

The only other libFLAC call (seek) does not produce fatal errors.

Call it in the function that also invokved
FLAC__stream_decoder_init_*().

https://www.xkcd.com/292/

Add GMutex, GCond attributes which will be used by callers to
conditionally wait on the stream.

Remove the (now-useless) plugin method buffer(), wait on GCond
instead.  Lock the input_stream before each method call.  Do the same
with the playlist plugins.