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/Theme.cpp

@@ -485,7 +485,7 @@ public:
    virtual ~SourceOutputStream();
 
 protected:
-   virtual size_t OnSysWrite(const void *buffer, size_t bufsize);
+   size_t OnSysWrite(const void *buffer, size_t bufsize) override;
    wxFile File;
    int nBytes;
 };