Restore ctkDICOMModel as a QAbstractItemModel

Otherwise ctkDICOMModelTest2 fails because ctkDICOMModel::fetch() doesn't
create a QStandardItemModel.

Libs/DICOM/Core/ctkDICOMModel.cpp

@@ -377,7 +377,7 @@ void ctkDICOMModelPrivate::fetch(const QModelIndex& indexValue, int limit)
 
 //------------------------------------------------------------------------------
 ctkDICOMModel::ctkDICOMModel(QObject* parentObject)
-  : QStandardItemModel(parentObject)
+  : Superclass(parentObject)
   , d_ptr(new ctkDICOMModelPrivate(*this))
 {
   Q_D(ctkDICOMModel);

Libs/DICOM/Core/ctkDICOMModel.h

@@ -28,9 +28,12 @@
 #include "ctkDICOMCoreExport.h"
 
 class ctkDICOMModelPrivate;
-class CTK_DICOM_CORE_EXPORT ctkDICOMModel : public QStandardItemModel
+class CTK_DICOM_CORE_EXPORT ctkDICOMModel
+//  : public QStandardItemModel
+  : public QAbstractItemModel
 {
   Q_OBJECT
+  typedef QAbstractItemModel Superclass;
 public:
 
   enum {