Added a ComboBoxDelegate for the new "type" role.

widgets/controls/comboboxdelegate.h

@@ -0,0 +1,31 @@
+#ifndef COMBOBOXDELEGATE_H
+#define COMBOBOXDELEGATE_H
+
+#include <QStyledItemDelegate>
+class QStringListModel;
+
+class ComboboxDelegate : public QStyledItemDelegate {
+  // TODO move source code files into subfolder "widgets/delegate"
+  Q_OBJECT
+ public:
+  explicit ComboboxDelegate(const QStringList items, QObject* parent = nullptr);
+
+  /// QAbstractItemDelegate interface
+ public:
+  void paint(QPainter* painter,
+             const QStyleOptionViewItem& option,
+             const QModelIndex& index) const override;
+  QSize sizeHint(const QStyleOptionViewItem& option, const QModelIndex& index) const override;
+  QWidget* createEditor(QWidget* parent,
+                        const QStyleOptionViewItem& option,
+                        const QModelIndex&) const override;
+  void setEditorData(QWidget* editor, const QModelIndex& index) const override;
+  void setModelData(QWidget* editor,
+                    QAbstractItemModel* model,
+                    const QModelIndex& index) const override;
+
+ private:
+  QStringListModel* m_types = nullptr;
+};
+
+#endif  // COMBOBOXDELEGATE_H