QSoundEffect: fix changing the loop count while playing.

The running count was not updated with the new value.

Auto-test added and documentation updated to be more clear about this
behavior.

Task-number: QTBUG-36643

Change-Id: I29e98ca4679f950a75133b21873738bcb72d23d4
Reviewed-by: Christian Stromme <christian.stromme@digia.com>