4.9 claims webengine sources are not part of any project

At the top of the editor window Qt Creator will warn that the current file is not part of any project, even though it is in the project sidebar.

The situation in WebEngine Core where this happens is that another build-system called GN has generated an intermediate file about its sources in the build dir, and the qmake file is including that and adding it to sources. Somewhere in the process one part of QtCreator now get lost and no longer considers the file part of the project.