Obsolete class CloseSongEnumerator, which was a kludge.

Since version 2.32, g_thread_init() is deprecated and a no-op.  Let's
upgrade that (optional) dependency so we can remove all those
g_thread_init() calls.

Auto-stop the IO thread in all error handlers.

Revert to the old API before commit e9e55b08, removing unnecessary
bloat.

Replaces the method Close().

Merge some duplicate code.

Avoid GLib.

Obsolete, our logging library doesn't ues GLib anymore.

Allows forward-declaration.

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

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

Replaces GLib's GError.

New GLib versions don't fail.

Deprecated.

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

Eliminates explicit global initialisation.

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.