sqlbrowser example: fix ugly margin around ConnectionWidget

When a layout is used to arrange the children of a non-top-level
custom widget, the layout's contentsMargins need to be manually set to
zero to avoid extra empty space around the widgets, misaligning it
w.r.t. its siblings.

Add the necessary call.

Amends 2690822428deec4f0c08f4d118d69a7c6036369e, which, however,
inherited the missing margin adjustment from older code.

Pick-to: 6.9 6.8
Change-Id: Icd1945a4f2b1635f031e50758ec2f1ec9313ae27
Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>

examples/sql/sqlbrowser/connectionwidget.cpp

@@ -14,6 +14,7 @@ ConnectionWidget::ConnectionWidget(QWidget *parent)
     , tree(new QTreeWidget(this))
 {
     QVBoxLayout *layout = new QVBoxLayout(this);
+    layout->setContentsMargins({});
     tree->setHeaderLabels(QStringList(tr("Database")));
     tree->header()->setStretchLastSection(true);
     QAction *refreshAction = new QAction(tr("Refresh"), tree);