Allow adding file from "This file is ot part of any project" bar

Use case:
1. Have an existing project
2. Copy a file to the project directory
3. Open this file with a Creator editor
4. The bar "This file is ot part of any project" appears at the top

Now it would be cool to have a button "Add to project" that allows adding this file easily. If multiple projects are open, there has to be selection dialog, of course.