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>

src/multimedia/audio/qaudioinput_alsa_p.h

@@ -126,6 +126,8 @@ public:
     QAudio::State state() const;
     void setFormat(const QAudioFormat& fmt);
     QAudioFormat format() const;
+    void setVolume(qreal);
+    qreal volume() const;
     bool resuming;
     snd_pcm_t* handle;
     qint64 totalTimeValue;
@@ -166,6 +168,7 @@ private:
     snd_pcm_format_t pcmformat;
     snd_timestamp_t* timestamp;
     snd_pcm_hw_params_t *hwparams;
+    qreal m_volume;
 };
 
 class InputPrivate : public QIODevice