Provide a nice way for developers to work with CMake-built Qt6 modules

Qt developers have a need to use Qt Creator to develop Qt 6.0 modules using the Qt6 CMake projects (CMakeLists.txt based projects).

Some of that desire was expressed offline, some of it online
https://lists.qt-project.org/pipermail/development/2020-June/039687.html
https://lists.qt-project.org/pipermail/development/2020-June/039678.html

This is probably a documentation issue as well as some changes needed in the CMake plugin.

Eike confirms that he was able to open qtbase and qtdeclarative with the steps outlined here:

https://lists.qt-project.org/pipermail/development/2020-June/039701.html

I tested a different list of steps (a longer one) that can be found here https://wiki.qt.io/CMake_Port/Qt_Creator (method 2)

This user story aims to facilitate discussion on what's missing and what can be improved.