Fix the bufferProgress property of QDeclarativeAudio

Fix the bufferProgress property of QDeclarativeAudio

The bufferProgress property of QDeclarativeAudio is documented to hold
how much of the data buffer is currently filled, from 0.0 (empty) to
1.0 (full) and should be declared to be of type qreal instead of int.
The implementation was correctly normalizing the buffer status
percentage to the [0, 1] range but the returned value was truncated
to int in QML because of the faulty property declaration.

Change-Id: I494c9cd14f4b3ec511759ae66a50e9f96bcb1452
Reviewed-by: Zeno Albisser <zeno.albisser@digia.com>