Use older, compatible "Qt::MidButton" rather than MiddleButton

Apparently Qt only added MiddleButton for Qt 4.7 so we use the older
style so we can compile on 4.6.

Libs/Widgets/ctkQImageViewerWidget.cpp

@@ -579,7 +579,7 @@ void ctkQImageViewerWidget::mousePressEvent( QMouseEvent * event )
         d->MouseLastIntensityWindowMax = this->intensityWindowMax();
         break;
         }
-      case Qt::MiddleButton:
+      case Qt::MidButton:
         {
         d->MouseMiddleDragging = true;
         d->MouseLastX = event->x();