NOTE: I don't know the reason for the class sysSettings to inherit the wxConfig privately in original implementation.
It was done by Dave in this commit: cfbae7d378766ab8cf138123186a024afffc061e.
That was a long time ago, but if memory serves it was intentionally done to force the use of the sysSettings members, and not wxConfig, because some of them have similar names (because there are only so many ways to name some generic functions), but do additional things that we need; and in the past some code had crept in that used wxConfig directly, bypassing our additional code and subtly breaking things.
Sending some 3 small fixes in separate emails for issues that me and my colleagues found really annoying. This is my first try at Pgadmin and I haven't done too much C++ though so in case something should be corrected let me know and I'd be happy to do that.