Add RelativePanel control

[dotMorten]

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 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).

[dotMorten]

With WinUI 3 source code finally available, I was able to port this control to WPF and verify it works exactly as the UWP/WinUI one. I'm more than happy to contribute it to this repro if you're interested.

[miloush]

It sounds like you are in a good position to make an API proposal!

[SlimeNull]

I've re-ported RelativePanel to WPF, and made some changes related to testing and performance improvements.

But I don't know how to submit code to WPF. Here is my ported RelativePanel, can anyone help me?

/~https://github.com/OrgEleCho/EleCho.WpfSuite/blob/master/EleCho.WpfSuite/Panels/RelativePanel.cs