Provide an equivalent for the WinForms Screen class

WPF generally doesn't include Windows Forms (the developer has to reference WinForms explicitly). We do this (among other reasons) so that we don't pollute IntelliSense with two UI stacks.

One common reason why people end up having to reference WinForms is so that they can enumerate screens and put windows on specific screens. But even with WinForms, the APIs aren't exactly super usable from WPF, due to mismatches in primitive types. It would be good if WPF would provide an equivalent type.