Add shortcut for select all in QSpinBox

This adds the shortcut to spinboxes and date edites.

Fixes: QTBUG-48231
Change-Id: I91400e1990e4b1d7d971198c1e2a64149c2a835e
Reviewed-by: Shawn Rutledge <shawn.rutledge@qt.io>

src/widgets/widgets/qabstractspinbox.cpp

@@ -1319,6 +1319,7 @@ void QAbstractSpinBox::contextMenuEvent(QContextMenuEvent *event)
     d->reset();
 
     QAction *selAll = new QAction(tr("&Select All"), menu);
+    selAll->setShortcut(QKeySequence::SelectAll);
     menu->insertAction(d->edit->d_func()->selectAllAction,
                       selAll);
     menu->removeAction(d->edit->d_func()->selectAllAction);