This adds a bit of overhead, but also adds flexibility to the API,
because arbitrary triggers may be invoked from that virtual method
implementation, not just Cond::signal().

The motivation for this is to make the handlers more dynamic, for the
upcoming buffering class utilizing ProxyInputStream.

Eliminate dependency on io_thread_get().

Prepare for a Decoder API redesign based on an abstract class with
virtual methods.

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.

Move code from the "mad" plugin.

Move code from the "mad" plugin.

Move and refactor code from the Wavpack decoder plugin.

Eliminate duplicate code.

Replaces the method Close().

Avoid GLib.

Allows building without GLib.  This fails to compile currently,
because GLib is still used in the MPD core.

Move code from the "mad" plugin.

Move code from the "mad" plugin.