Rewrite PulseAudio backend into a threaded class

Rewrite PulseAudio backend into a threaded class

Converts the Mixer PulseAudio backend from a C library to a PaClient
class which runs in its own QThread.  This faciliates isolation of
PaControlModel updates to the QML thread where they belong. It also
provides the foundation for runtime updates of the model and reuse
of the PaClient class in other apps.

AGL-Bug: SPEC-548
Change-Id: I13c4c220fde2fd4bc4aea2e04f39152a963b5fa0
Signed-off-by: Matt Porter <mporter@konsulko.com>