Change MidButton to MiddleButton due to Qt deprecation

This came from a warning while building on Ubuntu:

bookidxview.cpp:7:31: warning: ‘Qt::MidButton’ is deprecated:
MidButton is deprecated. Use MiddleButton instead
    7 |     _buttonRubberBandZoom(Qt::MidButton),

libkoviz/bookidxview.cpp

@@ -4,7 +4,7 @@ BookIdxView::BookIdxView(QWidget *parent) :
     QAbstractItemView(parent),
     _curvesView(0),
     _buttonSelectAndPan(Qt::LeftButton),
-    _buttonRubberBandZoom(Qt::MidButton),
+    _buttonRubberBandZoom(Qt::MiddleButton),
     _buttonResetView(Qt::RightButton)
 {
 }