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/Theme.cpp

@@ -477,7 +477,7 @@ void FlowPacker::RectMid( int &x, int &y )
 /// The trick used here is that wxWidgets can write a PNG image to a stream.
 /// By writing to a custom stream, we get to see each byte of data in turn, convert
 /// it to text, put in commas, and then write that out to our own text stream.
-class SourceOutputStream : public wxOutputStream
+class SourceOutputStream final : public wxOutputStream
 {
 public:
    SourceOutputStream(){;};