BaseTrackTableModel: fix Qt6 QVariant deprecation warning

[Be-ing]

../src/library/basetracktablemodel.cpp: In member function ‘QVariant BaseTrackTableModel::roleValue(const QModelIndex&, QVariant&&, int) const’:
../src/library/basetracktablemodel.cpp:649:30: warning: ‘QVariant::Type QVariant::type() const’ is deprecated: Use typeId() or metaType(). [-Wdeprecated-declarations]
  649 |             if (rawValue.type() == QVariant::String) {
      |                 ~~~~~~~~~~~~~^~
In file included from /usr/include/qt6/QtCore/qabstractitemmodel.h:47,
                 from /usr/include/qt6/QtCore/QAbstractTableModel:1,
                 from ../src/library/basetracktablemodel.h:3,
                 from ../src/library/basetracktablemodel.cpp:1:
/usr/include/qt6/QtCore/qvariant.h:337:10: note: declared here
  337 |     Type type() const
      |          ^~~~

[Holzhaus]

Thanks!