Hello Harald,
I suppose, that when working with C#/XAML, when I use Create New Item, I can only use those under C#/XAML? In other words: I cannot mix with other kinds of items.
Yes, you are right, when you add a new item to the project, you can only add the items under the "C#/XAML for HTML5" section of the "Add New Item" dialog of Visual Studio.
How do I create a ChildWindow similiar to in Silverlight? F.ex as a popup login window.
Support for popups and ChildWindow is coming in early 2016.
In the meantime, you can do the same as in the sample Showcase application, that is, to use a Grid and change the Visibility of the foremost element to simulate a popup. You can see it in action by launching the Showcase application: you will notice that the "Welcome" screen is like a non-movable child window. To see the source code of the Showcase application, click "New Project" -> "C#/XAML for HTML5" -> "Showcase".
Regards,
JS-Support