Don't use integers to describe volume internally in QSoundEffect.

The public api takes floating point values and so does most of the back-
ends. Conversion should be done in the back-ends that expect other value
types to avoid unnecessary float -> int -> float conversions.

Change-Id: I0ee1bfbe350f985294c20f897ffa3bd55288b4c9
Reviewed-by: Yoann Lopes <yoann.lopes@digia.com>