Consider switch to C++/WinRT for winrt port of Qt

C++/WinRT is the standard language projection (header only) for winrt. By using that projection technology, we could expect a major performance boost and code maintainability would be greatly increased. Check https://blogs.windows.com/buildingapps/2016/11/28/standard-c-windows-runtime-cwinrt/#Ll2x5XQdI8mct666.97 for some information about the benefits of this technology.

C++/WinRT became part of the Windows SDK in version 10.0.17134.0 (Windows 10, version 1803, April 2018). That means that we would have to increase the minimal version from our current version of 10.0.14393.0 (Windows 10, version 1607, August 2016). By the time Qt 6 arrives, the new version will have been released over 2 years ago.

Changing to C++/WinRT would mean a major rewrite of the current code base. Most of winrt specific code would have to be rewritten. This does not have to happen in a bulk change. The code base can be moved over to the new system incrementally.