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/LegacyAliasBlockFile.h

@@ -15,7 +15,7 @@
 #include "PCMAliasBlockFile.h"
 
 /// An AliasBlockFile that references uncompressed data in an existing file
-class LegacyAliasBlockFile : public PCMAliasBlockFile
+class LegacyAliasBlockFile final : public PCMAliasBlockFile
 {
  public:
 

src/blockfile/LegacyBlockFile.h

@@ -32,7 +32,7 @@ void ComputeLegacySummaryInfo(wxFileName fileName,
 // back to disk, but you can't create a NEW one from new
 // sample data.
 //
-class LegacyBlockFile : public BlockFile {
+class LegacyBlockFile final : public BlockFile {
  public:
 
    // Constructor / Destructor

src/blockfile/ODDecodeBlockFile.h

@@ -35,7 +35,7 @@ Also, see ODPCMAliasBlockFile for a similar file.
 #include <wx/thread.h>
 
 /// An AliasBlockFile that references uncompressed data in an existing file
-class ODDecodeBlockFile : public SimpleBlockFile
+class ODDecodeBlockFile final : public SimpleBlockFile
 {
  public:
 

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

src/blockfile/PCMAliasBlockFile.h

@@ -15,7 +15,7 @@
 #include "../DirManager.h"
 
 /// An AliasBlockFile that references uncompressed data in an existing file
-class PCMAliasBlockFile : public AliasBlockFile
+class PCMAliasBlockFile /* not final */ : public AliasBlockFile
 {
  public:
    /// Constructs a PCMAliasBlockFile, writing the summary to disk

src/blockfile/SilentBlockFile.h

@@ -21,7 +21,7 @@
 #include "../DirManager.h"
 
 /// A BlockFile containing nothing but silence.  Saves disk space.
-class SilentBlockFile : public BlockFile {
+class SilentBlockFile final : public BlockFile {
  public:
 
    // Constructor / Destructor

src/blockfile/SimpleBlockFile.h

@@ -42,7 +42,7 @@ typedef struct {
    wxUint32 channels;   // number of interleaved channels
 } auHeader;
 
-class SimpleBlockFile : public BlockFile {
+class SimpleBlockFile /* not final */ : public BlockFile {
  public:
 
    // Constructor / Destructor