Using the new SpinBoxDelegate in int and double columns even if the cell is empty.

widgets/controls/spinboxdelegate.h

@@ -0,0 +1,27 @@
+#ifndef SPINBOXDELEGATE_H
+#define SPINBOXDELEGATE_H
+
+#include <QStyledItemDelegate>
+
+class SpinboxDelegate : public QStyledItemDelegate {
+  // TODO move source code files into subfolder "widgets/delegate"
+  Q_OBJECT
+ public:
+  explicit SpinboxDelegate(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& index) const override;
+  void setEditorData(QWidget* editor, const QModelIndex& index) const override;
+  void setModelData(QWidget* editor,
+                    QAbstractItemModel* model,
+                    const QModelIndex& index) const override;
+};
+
+#endif  // SPINBOXDELEGATE_H