Add another menu action to the Qt Quick Controls application template

We have had a few bug reports about a missing menu bar on Mac (i.e QTBUG-37517). The real issue is that the default template only contains an "exit" action, which is automatically moved to the application menu on Mac and hence the "File" menu dissapears.

It can easily be solved if we add a second action such as

 MenuItem {  text: qsTr("&Open") ; onTriggered: Console.log("Open action triggered") }

Note while we could add a MessageBox here, it would depend on 5.3.