Handle signals through XML files

A while ago, I solved the problem with default parameters in signals.

Since qt5, there is the “dataChanged(QModelIndex,QModelIndex,QVector<int>)”
signal, where the last argumend is optional.

This had to be supported in PySide2.

I searched quite long and implemented a lot of functions in the XML files, but
that did not work at all, since signals are handled specially in ‘pysidemetafunction.cpp’.

Finally, I implemented a default argument for this single case. It has to be written
in a more general way eventually, when we have more such cases.
For now, the default argument problem is solved.

General consideration:
Should we not better implement signals like all the other methods in XML files?
Why was it done directly in 'pysidemetafunction.cpp', is there any reason but history?
I think this should be visible in the XML files, and I would like to re-write
my ad-hoc solution.