Reduce header dependencies.

The plugin assumes that the size is known, but does not verify it at
runtime.

Remove the "whence" parameter that is not actually necessary, and only
complicates the InputStream implementations.

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

Replaces GLib's GError.

Hide the definition from C code, to prepare the transition to C++.

This might break older versions, I didn't test.

This might break older versions, I didn't test.

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

The stdio header is no longer pulled in by af_vfs.h in audiofile-0.3.0.

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.

Remove trailing whitespace found by this command:
find -name '*.[ch]' | xargs grep "[[:space:]]$"

Make it X_decoder_plugin.c.