Organizing source code in hierachical subfolders is, in my humble opinion, a mandatory task for keeping code tidy, structured and comprehensible.
Due to this I was pretty astonished about Qt Creator's lack of an intuitive procedure to create and display subfolders. I would have expected to find an option like "create subdirectory here" next to "Add New..." in the context menu of my project, but was disappointed.
A little research lead me to a stackoverflow thread with solutions to the aforementioned problem.
For moving an existing file into a subdirectory one has to create that subdirectory beforehand. Subsequently the file has to be renamed to subdirectory/file. When creating new files it is possible to specify a subdirectory the same way in the Header / Source file field.
Those are obviously not the most intuitive interactions that one could have thought of, but they get the job done.
I hope this may have been of help for someone out there.