... Should have no effect on generated code, except perhaps some slight faster
virtual function calls. Mostly useful as documentation of design intent.
Tried to mark every one of our classes that inherits from another, or is a
base for others, or has abstract virtual functions, and a few others besides.
... in the first label track.
Problem was calling wxTextCtrl::AppendText once per label, per push of undo
stack, each call causing event handling.
Now call it only once per push.
