fix overwriting of properties in QPrintEngine

This was introduced in 5568620e334578818215b378cdbbb632719907a6 and in
ca895456174a334dda1ae051a8f51b675f196b8a, where custom flags were added
in QPrintEngine::PrintEnginePropertyKey. However, they were introduced
after PPK_PaperSize, which was aliased to PPK_PageSize -- which caused
the enum counting to reset to the earlier defined value. Hence the
numeric values for the custom enums were duplicated -- which caused
problems when not using the PDF printing engine.