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/Shuttle.h

@@ -56,7 +56,7 @@ public:
    wxString mParams;
    ShuttleCli(){ mParams = wxT("") ;}
    virtual ~ShuttleCli() {}
-   virtual bool ExchangeWithMaster(const wxString & Name);
+   bool ExchangeWithMaster(const wxString & Name) override;
 };
 
 #endif