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

Use a listener interface instead of GlobalEvents.

Passing a void pointer is unsafe.

Replaces GLib's GError.

After we've been hit by Large File Support problems several times in
the past week (which only occur on 32 bit platforms, which I don't
have), this is yet another attempt to fix the issue.

It's possible to have a mixer implementation which does not explicitly
need the methods open() and close().

This allows the mixer object to access its associated audio output
object.

This mixer plugin may be used instead of the traditional global
software mixer.  It integrates with the "volume" filter plugin, and
can control the software volume of an audio output which has no
hardware mixer.