Added volume control for QAudioOutput & QAudioInput (alsa)

QTBUG-25454

- Added update to docs on volume control.
- Added internal volume adjustment for alsa implementation.
- Enabled float sample option in QAudioDeviceInfo (alsa).

Change-Id: I6b89fc8beb457d71be9ad71b538c86a008570f07
Reviewed-by: Michael Goddard <michael.goddard@nokia.com>
Reviewed-by: Kurt Korbatits <kurt.korbatits@nokia.com>