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

The attribute must be 0..100 and not 0..1024.  Previously, the code
was inconsistent.

-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.

Another big chunk of code for multi-player support.

Move code from AlsaMixerPlugin.

EventLoop::AddCall() and EventLoop::AddIdle() are unsafe, because we
can't cancel those calls.

This deadlock was a regression by commit 8e38b4f8.  Since we currently
can't resolve this, let's revert the commit, and add a GLib specific
workaround for the build failure.

This is safer, and works without epoll().  Fixes a build failure with
uClibc, which does not support epoll().