Change ctkDICOMModel to be a QStandardItemModel

In order to set the checkable status of items, need to have Standard
instead of Abstract model inheritance.

Libs/DICOM/Core/ctkDICOMModel.cpp

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

Libs/DICOM/Core/ctkDICOMModel.h

@@ -22,13 +22,13 @@
 #define __ctkDICOMModel_h
 
 // Qt includes 
-#include <QAbstractItemModel>
+#include <QStandardItemModel>
 #include <QSqlDatabase>
 
 #include "ctkDICOMCoreExport.h"
 
 class ctkDICOMModelPrivate;
-class CTK_DICOM_CORE_EXPORT ctkDICOMModel : public QAbstractItemModel
+class CTK_DICOM_CORE_EXPORT ctkDICOMModel : public QStandardItemModel
 {
   Q_OBJECT
 public: