Move IEditor::createNew to IDocument::setContents

The method is for setting the contents, so it belongs to the document,
and should be named correspondingly.

Change-Id: I40363dc08f11268f530885b512e4a88e8b10d096
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>
Reviewed-by: Eike Ziller <eike.ziller@digia.com>