Widgets snippets: Use the header file instead of a new class each time

In several snippets, the TableModel class is created and used.
I included a header file to avoid code repetition.

Change-Id: Ie6318606cc89a2f5f1c137a36195fe290b6c1be6
Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>

src/widgets/doc/snippets/itemselection/main.cpp

@@ -13,29 +13,7 @@
 #include <QItemSelectionModel>
 #include <QTableView>
 
-class TableModel : public QAbstractTableModel
+#include "../common-table-model/model.h"
-{
-    Q_OBJECT
-public:
-    TableModel(int rows, int columns, QObject *parent = nullptr)
-        : QAbstractTableModel(parent), m_rows(rows), m_columns(columns) {}
-
-    int rowCount(const QModelIndex &parent = QModelIndex()) const override {
-        return m_rows;
-    }
-
-    int columnCount(const QModelIndex &parent = QModelIndex()) const override {
-        return m_columns;
-    }
-
-    QVariant data(const QModelIndex &index, int role = Qt::DisplayRole) const override {
-            return QVariant();
-    }
-
-private:
-    int m_rows, m_columns;
-
-};
 
 int main(int argc, char *argv[])
 {

src/widgets/doc/snippets/reading-selections/window.cpp

@@ -17,25 +17,7 @@
 #include <QTableView>
 
 #include "../include/mainwindow.h"
-
+#include "../common-table-model/model.h"
-class TableModel : public QAbstractTableModel
-{
-    Q_OBJECT
-public:
-    TableModel(int rows, int columns, QObject *parent = nullptr) {}
-
-    int rowCount(const QModelIndex &parent = QModelIndex()) const override {
-        return 0;
-    }
-
-    int columnCount(const QModelIndex &parent = QModelIndex()) const override {
-        return 0;
-    }
-
-    QVariant data(const QModelIndex &index, int role = Qt::DisplayRole) const override {
-        return QVariant();
-    }
-};
 
 MainWindow::MainWindow(QWidget *parent)
     : QMainWindow(parent)

src/widgets/doc/snippets/sharedtablemodel/main.cpp

@@ -13,25 +13,7 @@
 #include <QItemSelectionModel>
 #include <QTableView>
 
-class TableModel : public QAbstractTableModel
+#include "../common-table-model/model.h"
-{
-    Q_OBJECT
-public:
-    TableModel(int rows, int columns, QObject *parent = nullptr)
-        : QAbstractTableModel(parent), rowCount(rows), columnCount(columns) {}
-
-    int rowCount(const QModelIndex &parent = QModelIndex()) const override {
-        return rowCount;
-    }
-
-    int columnCount(const QModelIndex &parent = QModelIndex()) const override {
-        return columnCount;
-    }
-
-    QVariant data(const QModelIndex &index, int role = Qt::DisplayRole) const override {
-        return QVariant();
-    }
-};
 
 int main(int argc, char *argv[])
 {

src/widgets/doc/snippets/updating-selections/window.cpp

@@ -15,26 +15,7 @@
 #include <QTableView>
 
 #include "../include/mainwindow.h"
-
+#include "../common-table-model/model.h"
-class TableModel : public QAbstractTableModel
-{
-    Q_OBJECT
-public:
-    TableModel(int rows, int columns, QObject *parent = nullptr)
-        : QAbstractTableModel(parent) {}
-
-    int rowCount(const QModelIndex &parent = QModelIndex()) const override {
-        return 0;
-    }
-
-    int columnCount(const QModelIndex &parent = QModelIndex()) const override {
-        return 0;
-    }
-
-    QVariant data(const QModelIndex &index, int role = Qt::DisplayRole) const override {
-        return QVariant();
-    }
-};
 
 MainWindow::MainWindow(QWidget *parent)
     : QMainWindow(parent)