WinUI support

Problem to solve

Lots of those.

• How does that integrates with existing UWP code?
• Uno relation?
• 3.x and 4.x branch differences?
• MediaElement.

Do we use

• the HWND API (can we? on WinUI on win32 maybe?)
• the current swapchain hack (better if we don't).
• the new output callbacks API (that's libvlc 4 only)

And plenty more questions...

Intended users

Windows users.

Proposal

Probably we would aim for WinUI 3

Should we aim for a net5 target?

WinUI is claimed to be compatible with C++/WinRT. If we make a WinUI control with libvlcsharp, can it be used (even, the control only) by C++ code? That's not clear (#147)

• https://github.com/microsoft/microsoft-ui-xaml/blob/master/docs/roadmap.md
• https://github.com/microsoft/microsoft-ui-xaml-specs/blob/9c72d8dde073924516b125e1ddaab1de36c61bc4/active/Win32/Window_and_Application_API_Spec.md#get-the-underneath-window-implementation
• https://github.com/microsoft/microsoft-ui-xaml/issues/3859
• https://github.com/microsoft/microsoft-ui-xaml/blob/9672592eba2a877718700c79b858ccdad69b65f3/dev/WebView2/WebView2.cpp#L1451

Related #346

Documentation

Yep lots of these.

/cc @kakone, @jeremyVignelles