WinRT: Investigate enabling windows QPA plugin as a development convenience

As a convenience for developers, and possibly a path forward in improving the use of modern Windows APIs in Qt, the windows QPA plugin could be built as a secondary plugin for WinRT builds. This way, apps could run on the Desktop but use the same Qt libraries that the rest of the port uses.

The WinRT port should be compatible with Desktop Windows 8, as the nearly all incompatible APIs should be contained within the QPA plugin. The windows plugin would be compiled with WINAPI_FAMILY_DESKTOP_APP, enabling the use of Win32 APIs. The imported classes (such as the core event dispatcher) will need to be compiled into the plugin in this special case, but the changes needed to Windows QPA are likely minimal.