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/blockfile/ODPCMAliasBlockFile.h

@@ -43,7 +43,7 @@ Some of these methods have been overridden only because they used the unsafe wxL
 #include <wx/thread.h>
 
 /// An AliasBlockFile that references uncompressed data in an existing file
-class ODPCMAliasBlockFile : public PCMAliasBlockFile
+class ODPCMAliasBlockFile final : public PCMAliasBlockFile
 {
  public:
    /// Constructs a PCMAliasBlockFile, writing the summary to disk