Large C++ files generated from .slint files can cause link problems with MSVC.
When that happens, MSVC itself suggests /bigobj as remedy.
It's not apparent that there's a downside to using this option, apart from
backwaords binary compatibility - which is not a concern for us.
The discussion at https://developercommunity.visualstudio.com/t/Enable-bigobj-by-default/1031214 suggests that perhaps this
will become the default one day.
To simplify our examples and avoid users running into this, let's enable it by default.