Investigate integration of "Windows App SDK" functionality into Qt

More and more functionality on Windows is only made available via Microsoft's "Windows App SDK". Some of this functionality is relevant is relevant for Qt development itself - like the possibility to customize windows' title bars or push notifications.

At the time of writing some hurdles to get Windows App SDK functionality into Qt are:

• no cmake support out of the box
  • There are some workarounds but most functionality is only documented from an msbuild point of view and there is no cmake support
  • As this is Windows only functionality, interest seems to be low
• Windows App SDK functionality relies on availability of c++/winrt and thus might not be available when using MinGW or clang
• In their documentation, Microsoft pulls in Windows App SDK functionality via nuget. We have to mimic the behavior and maybe reverse engineer the needed steps to be done in cmake.
• For "unpackaged libraries and applications" (which most Qt apps are) the app must "initialize the Windows App SDK runtime before using any other feature of the Windows App SDK." which seems to be an elaborate process we have to mimic in Qt's/the app's build system.