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

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Follow-up of #850 and #858