add qml.SetWindowIcon function

I think that a Application should have a custom icon to look different.
But I don't find an access to deal with it now.
So I add the function.
If there is a way to add the application icon, you can close this request.
And tell me what to do.
I'm a newbie on github, I don't know how to pull anothor request in same branch or update pull request in that response. So I create another brach :(