Allows forward-declaration.

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

Replaces GLib's GError.

New GLib versions don't fail.

Avoid duplicating the tag.

Deprecated.

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

Eliminates explicit global initialisation.

Some compilers are very picky, but we really aren't interested in the
return value.

Some compilers are very picky, but we really aren't interested in the
return value.

The other audio_format attributes are not used.

Needed for the I/O thread.

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.

To demonstrate the new I/O thread.  libsoup is well-integrated into
the GLib main loop, which made this plugin pretty easy to write.

As a side effect, we have to initialize the I/O thread in all debug
programs that use the input API.