You can drag & drop source file to project. But be caution that when you want to create a folder, you must right click the project name -> new -> Source Folder. Other wise, the file in that folder want be compiled by IDE.
When I copied the source folder from the example to my project, it turns to a normal folder. So I tried to follow your steps to create a source folder. But the problems remain the same. I think there might be other causes):