Replace virtual with override wherever possible; eliminate needless virtual...

... for functions in final classes.

override is like const -- it's not necessary, but it helps the compiler to
catch mistakes.

There may be some overriding functions not explicitly declared virtual and I did
not identify such cases, in which I might also add override.

src/xml/XMLFileReader.h

@@ -18,7 +18,7 @@
 class AUDACITY_DLL_API XMLFileReader final {
  public:
    XMLFileReader();
-   virtual ~XMLFileReader();
+   ~XMLFileReader();
 
    bool Parse(XMLTagHandler *baseHandler,
               const wxString &fname);

src/xml/XMLWriter.h

@@ -78,7 +78,7 @@ class AUDACITY_DLL_API XMLFileWriter final : public wxFFile, public XMLWriter {
    void CloseWithoutEndingTags(); // for auto-save files
 
    /// Write to file. Might throw XMLFileWriterException.
-   void Write(const wxString &data);
+   void Write(const wxString &data) override;
 
  private:
 
@@ -107,7 +107,7 @@ class XMLStringWriter final : public wxString, public XMLWriter {
    XMLStringWriter(size_t initialSize = 0);
    virtual ~XMLStringWriter();
 
-   void Write(const wxString &data);
+   void Write(const wxString &data) override;
 
    wxString Get();