Examples: Revamp sql/books

* use nullptr
* use member init
* set ExpandingFieldsGrow fieldGrowthPolicy for QFormLayout, which
makes it behaviors similar on macOS as other platforms
* select first row to make up/down keys works by default

Pick-to: 6.5 6.6
Change-Id: I25d9869d2ca1c7274c2b750aada8270734787546
Reviewed-by: Liang Qi <liang.qi@qt.io>

examples/sql/books/bookwindow.cpp

@@ -89,6 +89,7 @@ BookWindow::BookWindow()
             );
 
     ui.bookTable->setCurrentIndex(model->index(0, 0));
+    ui.bookTable->selectRow(0);
     createMenuBar();
 }
 

examples/sql/books/bookwindow.h

@@ -22,8 +22,8 @@ private slots:
 private:
     void showError(const QSqlError &err);
     Ui::BookWindow ui;
-    QSqlRelationalTableModel *model;
-    int authorIdx, genreIdx;
+    QSqlRelationalTableModel *model = nullptr;
+    int authorIdx = 0, genreIdx = 0;
 
     void createMenuBar();
 };

examples/sql/books/bookwindow.ui

@@ -64,6 +64,9 @@
           <string>Details</string>
          </property>
          <layout class="QFormLayout">
+          <property name="fieldGrowthPolicy">
+           <enum>QFormLayout::ExpandingFieldsGrow</enum>
+          </property>
           <item row="0" column="0">
            <widget class="QLabel" name="label_5">
             <property name="text">