QTableView: fix cornerWidget connection

A click on the corner widget should select all items, not reset the
selection. This was accidentally added with
3e144bdc7484968dcccbda8f35ad802c7fd7a42e.

Fixes: QTBUG-124267
Change-Id: Ie20b7cf0ff730214dca80116ad888f42c2b7a670
Reviewed-by: Axel Spoerl <axel.spoerl@qt.io>
(cherry picked from commit 56ec0f94b3f9db19d5fe78d78dc7597a92a5c52f)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/widgets/itemviews/qtableview.cpp

@@ -595,7 +595,7 @@ void QTableViewPrivate::init()
     cornerWidget->setFocusPolicy(Qt::NoFocus);
     cornerWidgetConnection = QObject::connect(
           cornerWidget, &QTableCornerButton::clicked,
-          q, &QTableView::reset);
+          q, &QTableView::selectAll);
 #endif
 }