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/qaudiohelpers_p.h

@@ -0,0 +1,67 @@
+/****************************************************************************
+**
+** Copyright (C) 2012 Nokia Corporation and/or its subsidiary(-ies).
+** Contact: http://www.qt-project.org/
+**
+** This file is part of the Qt Toolkit.
+**
+** $QT_BEGIN_LICENSE:LGPL$
+** GNU Lesser General Public License Usage
+** This file may be used under the terms of the GNU Lesser General Public
+** License version 2.1 as published by the Free Software Foundation and
+** appearing in the file LICENSE.LGPL included in the packaging of this
+** file. Please review the following information to ensure the GNU Lesser
+** General Public License version 2.1 requirements will be met:
+** http://www.gnu.org/licenses/old-licenses/lgpl-2.1.html.
+**
+** In addition, as a special exception, Nokia gives you certain additional
+** rights. These rights are described in the Nokia Qt LGPL Exception
+** version 1.1, included in the file LGPL_EXCEPTION.txt in this package.
+**
+** GNU General Public License Usage
+** Alternatively, this file may be used under the terms of the GNU General
+** Public License version 3.0 as published by the Free Software Foundation
+** and appearing in the file LICENSE.GPL included in the packaging of this
+** file. Please review the following information to ensure the GNU General
+** Public License version 3.0 requirements will be met:
+** http://www.gnu.org/copyleft/gpl.html.
+**
+** Other Usage
+** Alternatively, this file may be used in accordance with the terms and
+** conditions contained in a signed written agreement between you and Nokia.
+**
+**
+**
+**
+**
+**
+** $QT_END_LICENSE$
+**
+****************************************************************************/
+
+#ifndef QAUDIOHELPERS_H
+#define QAUDIOHELPERS_H
+
+//
+//  W A R N I N G
+//  -------------
+//
+// This file is not part of the Qt API.  It exists purely as an
+// implementation detail.  This header file may change from version to
+// version without notice, or even be removed.
+//
+// We mean it.
+//
+
+#include <qaudioformat.h>
+
+QT_BEGIN_NAMESPACE
+
+namespace QAudioHelperInternal
+{
+void qMultiplySamples(qreal factor, const QAudioFormat& format, const void *src, void* dest, int len);
+}
+
+QT_END_NAMESPACE
+
+#endif