Harmlessly qualify classes as final (or explicitly comment not)...

... 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.

src/widgets/Warning.cpp

@@ -31,7 +31,7 @@ the ability to not see similar warnings again for this session.
 #include <wx/sizer.h>
 #include <wx/stattext.h>
 
-class WarningDialog : public wxDialog
+class WarningDialog final : public wxDialog
 {
  public:
    // constructors and destructors