Warn about `UIBarButtonItem` and similar ctors

[jonathanpeppers]

I don't think anything warns about code like this:

class MyView : UINavigationBar
{
    public class MyView()
    {
         Items = new[] { new UIBarButtonItem(OnButton) };
    }

    void OnButton(object sender, EventArgs e)
    {
    }
}

Note: I don't know if this code is exactly correct, this is from memory.