[qtmozembed] Added private window attribute to QuickMozView

In the backend API (https://github.com/tmeshkova/gecko-dev/blob/d95ddb24e6511215672cebb6103d32b0ae7b8615/embedding/embedlite/EmbedLiteApp.cpp#L396) we are referring to it as private window. Is it fine if there are naming differences between here and backend APIs ?