Alternative way to make signals private

By using QPrivateSignal as an extra arg to ensure users and sub-classes
can't emit them, c.f.:
https://woboq.com/blog/how-qt-signals-slots-work-part2-qt5.html

Note that this is needed to make it compile at all with the PMF signal/slot
syntax, if it's a private method we can't call it in the lambda.