the death of GdbCommandType & co.

instead of passing callback codes and dispatching them in a huge switch
statement, pass around member function pointers as real callbacks.
some codes which did not actually invoke a callback were completely
eliminated. hope this doesn't impair debugging. ;)
as a "side effect", command categorization by code is gone - there is a
proper flag field for that now.
as a real side effect, some suppressed assert failures might be back
now. fix them properly if they do ...