EditorManager: Open files with correct case

On case-insensitive file systems we still want files to open with the
correct case, because otherwise Qt Creator will later save it with a
different case.

Task-number: QTCREATORBUG-6510
Change-Id: I8e3f011939306d53c92e28fbf6eded1a2d42a9f2
Reviewed-by: Orgad Shaneh <orgads@gmail.com>