Fix breakage from commit bea5681f

Prepare to add an abstract class AudioOutput, to be implemented by
plugins, to get rid of the C-style vtable.

Make all the locks exception-safe.

The old struct config_param remains only for top-level string options.

-fixes regression introduced by:
 '8d6fedf8 [PATCH] Mixer: add class MixerListener'
-listener.OnMixerVolumeChanged() called GetVolume() which attempted to acquire
the lock but as per 'pa_threaded_mainloop_lock()' documentation:
This function may not be called inside the event loop thread. Events that are
dispatched from the event loop thread are executed with this lock held
-this patch seperates the underlying action of GetVolume() into a new
GetVolumeInternal() function, to be called only when the lock is already held, as
is the case for the listener.OnMixerVolumeChanged() call

Use a listener interface instead of GlobalEvents.

Passing a void pointer is unsafe.

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.

Provide _lock() and _unlock() to wrap all accesses from the mixer
plugin.