Fix/replace qtdemo launcher

From the user viewpoint, the lack of the old qtdemo launcher makes it much harder to see what demos are available, to find demos for a specific feature, to find docs for a demo, and to run demos on Windows (because you can't run them manually without setting the path to the Qt libs in the environment).

From the tester's viewpoint, the lack of the qtdemo launcher makes it take significantly longer to perform package testing and makes it more difficult to compare multiple Qt versions for regressions.

It would be great if we could get qtdemo working again for the modularized Qt or come up with a suitable replacement.