QML Only + C++: Add support for Qt6.

During the Lomiri CodeFest in Tilburg, I played with clickable and bootstrapped an example app. I then tinkered with the resulting app created from template and wanted to see what is needed to port the generated app to Qt6 / QMLv6. Here is the result poured back into the templating engine. (Note, that the code tree generation is untested, as I have no clue how that would be done with clickable (or the template tooling it uses)).