QWheelEvent: make magic number 120 a symbolic constant

The documentation states that 120 is the value users of
the event should use to determine one full step of the
wheel. Provide that number as a symbolic constant.

Change-Id: I0da0cdd8328a476538080b7276d02863906ea53f
Reviewed-by: Shawn Rutledge <shawn.rutledge@digia.com>

src/gui/kernel/qevent.h

@@ -170,6 +170,8 @@ protected:
 class Q_GUI_EXPORT QWheelEvent : public QInputEvent
 {
 public:
+    enum { DefaultDeltasPerStep = 120 };
+
     QWheelEvent(const QPointF &pos, int delta,
                 Qt::MouseButtons buttons, Qt::KeyboardModifiers modifiers,
                 Qt::Orientation orient = Qt::Vertical);