TextEditor: Normalize path on Windows

Searching with wrong case causes the code-model to have duplicate files
when opening a file from the results

Change-Id: Id3e4d875230e14c2ab23345a7440372f71f76e8b
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>
Reviewed-by: David Schulz <david.schulz@digia.com>