Command validator cloners return smart pointers, and...

... defined DefaultValidator so Validator can be abstract, as the comment for
Clone() stongly implies it should be.

But this is virtual is unused except in comments.

src/commands/SelectCommand.cpp

@@ -45,7 +45,7 @@ void SelectCommandType::BuildSignature(CommandSignature &signature)
    IntValidator *lastTrackValidator = new IntValidator();
    signature.AddParameter(wxT("LastTrack"), 0, lastTrackValidator);
 
-   Validator *trackNameValidator = new Validator();
+   Validator *trackNameValidator = new DefaultValidator();
    signature.AddParameter(wxT("TrackName"), 0, trackNameValidator);
 }