Add setWindowFlags to QMainWindowWrapper

[maciek134]

Added setWindowFlags to QMainWindowWrapper, so we can have stuff like dialogs and chromeless windows.

I see that I have a newer Qt version (5.15.0 vs 5.9.5) but the MOC version is the same, so it should be alright.

EDIT: well, Qt version may be alright, but it also uses newer C++ features that the msbuild on the CI doesn't support.