Allow to change the volume using a mouse hires wheel

The Volume control plugin allows to change the volume using
the mouse wheel. However for hires wheel this doesn't work.

The volume change is controlled by the following formula:

 m_volumeSlider->setSliderPosition(m_volumeSlider->sliderPosition()
   + (event->angleDelta().y() / QWheelEvent::DefaultDeltasPerStep *
   m_volumeSlider->singleStep()));

For some kind of mouse where the wheel event is compose by a lot
of small 'delta'; the ratio

   event->angleDelta().y() / QWheelEvent::DefaultDeltasPerStep

is less than 1; and because this ratio is between two integers
it is rounded to 0.

So store the fraction part of the wheel stroke in a static variable.

plugin-volume/volumepopup.cpp

@@ -197,8 +197,21 @@ void VolumePopup::openAt(QPoint pos, Qt::Corner anchor)
 
 void VolumePopup::handleWheelEvent(QWheelEvent *event)
 {
-    m_volumeSlider->setSliderPosition(m_volumeSlider->sliderPosition()
-            + (event->angleDelta().y() / QWheelEvent::DefaultDeltasPerStep * m_volumeSlider->singleStep()));
+    // With a hig res mouse wheel, event->angleDelta().y() may be
+    // less than QWheelEvent::DefaultDeltasPerStep, so we need to
+    // accomulate the fraction between each handleWheelEvent call.
+    // We can declare it static, because even if it would another
+    // instance of VolumePopup, this would impact onlythe start
+    // of the strokes.
+    static int fractionalAngleDelta = 0;
+
+    fractionalAngleDelta += event->angleDelta().y();
+    if (fractionalAngleDelta / QWheelEvent::DefaultDeltasPerStep != 0) {
+        m_volumeSlider->setSliderPosition(m_volumeSlider->sliderPosition()
+            + (fractionalAngleDelta / QWheelEvent::DefaultDeltasPerStep *
+	    m_volumeSlider->singleStep()));
+	fractionalAngleDelta %= QWheelEvent::DefaultDeltasPerStep;
+    }
 }
 
 void VolumePopup::setDevice(AudioDevice *device)