QMakePM: Deduplicate comparison

Change-Id: I687e5cef4eda5baf20c46cda1bb41882c952a849
Reviewed-by: Fawzi Mohamed <fawzi.mohamed@digia.com>
DiffEditor: Pass const lists and string by reference

Change-Id: I6acdc4db61aeed1353bed2207736268dfabf3f17
Reviewed-by: Jarek Kobus <jaroslaw.kobus@digia.com>