New File Wizard works with projects whose source dir is the root dir

It is not possible to create an instance of `IFolder` from a project path.
However, this behavior is required to create a file in a source dir that
is the same as a project dir. Therefore we switched from `IFolder` to
`IContainer` in the traits `NewFileWizard` and `FileCreator`.

Fixes #1002332
Merge pull request #871 from sschaef/t1002332-wizard-root-source-folders

New File Wizard works with projects whose source dir is the root dir