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/commands/AppCommandEvent.h

@@ -34,7 +34,7 @@ public:
    AppCommandEvent(const AppCommandEvent &event);
    ~AppCommandEvent();
 
-   virtual wxEvent *Clone() const;
+   wxEvent *Clone() const override;
    void SetCommand(Command *cmd);
    Command *GetCommand();