C++: handle case-insensitive file names in the WorkingCopy

... by keying on Utils::FileName

Task-number: QTCREATORBUG-12390
Change-Id: Ia1a59d2e422b4f92300ac6a8e0e52dd7456a6e70
Reviewed-by: Eike Ziller <eike.ziller@theqtcompany.com>
Reviewed-by: Nikolai Kosjar <nikolai.kosjar@theqtcompany.com>