Add RelativePanel control

I'd like to propose porting UWP's RelativePanel control to WPF. It's a great way to layout UI, while reducing the number of panels needed to create in order to perform such a layout. A panel like this could also form the basis for porting Adaptive Triggers down the line (as these two go really well hand-in-hand).

I have an implementation of it here that I'd be happy to contribute back to WPF:
https://github.com/dotMorten/UniversalWPF/tree/master/src/UniversalWPF.RelativePanel

I'm also hoping that the [Windows UI Xaml](https://github.com/Microsoft/microsoft-ui-xaml/issues/36) project will soon share UWP's RelativePanel implementation, to ensure 100% consistency in behavior (AFAIK my implementation matches in behavior, but knowing that there are a lot of edge cases, I probably don't handle them all).