Changes to GStreamer backend for audio decoder.

Changes to GStreamer backend for audio decoder.

Removed WaitingState.
New signals: finished(), positionChanged(), durationChanged().
New methods: position(), duration().
A parameter removed from read() method.
Unit tests updated.

Change-Id: Ie9d8a2804285c5542e592cce69963adbdf6ebfb8
Reviewed-by: Jonas Rabbe <jonas.rabbe@nokia.com>
Reviewed-by: Michael Goddard <michael.goddard@nokia.com>