Use placeholder text for accessible description of QLineEdit

If the accessible description of a QLineEdit is requested and none has been
set, the placeholder text will be provided if one is available.

Change-Id: I4c0dad1d06fd3d8a6e00c963402d380c59bd7f05
Reviewed-by: Jesus Fernandez <Jesus.Fernandez@qt.io>
Reviewed-by: Frederik Gladhorn <frederik.gladhorn@qt.io>

src/widgets/accessible/simplewidgets.cpp

@@ -704,6 +704,8 @@ QString QAccessibleLineEdit::text(QAccessible::Text t) const
     }
     if (str.isEmpty())
         str = QAccessibleWidget::text(t);
+    if (str.isEmpty() && t == QAccessible::Description)
+        str = lineEdit()->placeholderText();
     return str;
 }