QmlDesigner: Optimize isList method by using QStringView for expression

- Changed the expression variable type from QString to QStringView for
better performance.

Change-Id: I8cf8688615c126f281089f2c2b124b045765db8b
Reviewed-by: Thomas Hartmann <thomas.hartmann@qt.io>

src/plugins/qmldesigner/libs/designercore/model/bindingproperty.cpp

@@ -151,7 +151,9 @@ bool BindingProperty::isList() const
     if (!isValid())
         return false;
 
-    return expression().startsWith('[') && expression().endsWith(']');
+    QStringView expression = this->expression();
+
+    return expression.startsWith('[') && expression.endsWith(']');
 }
 
 QList<ModelNode> BindingProperty::resolveListToModelNodes() const