Overhaul/clean up handling of embedding of (semi)transparent windows on Windows

The topic of embedding (semi)transparent windows on Windows comes up again and again in bug reports. Instead of fixing bugs and causing regressions with these fixes we should see if we should overhaul the whole topic on Windows. That means:

• Having a uniform and deterministic way of embedding windows into windows on Windows ("native" windows into Qt windows as well as the other way around)
  • This should also cover "special cases" like ActiveQt
• Research on how to increase test coverage. Might be crossplatform auto test or Windows specific tests which catch regressions