Make wxValidator::SetWindow() virtual

Allow overriding the method called when the validator is associated with
the window, this can be convenient to perform some initialization on the
validator instance actually used as it can't be done on the initially
created object itself because it will be cloned by SetValidator(),
creating a new instance.

Also change SetWindow() to take wxWindow instead of wxWindowBase, this
still requires the cast in wxWindow::SetValidator(), but it's better to
have it there rather than in wxValidator and use the simpler type in the
public function signature.

src/common/wincmn.cpp

@@ -1782,10 +1782,10 @@ void wxWindowBase::SetValidator(const wxValidator& validator)
     if ( m_windowValidator )
         delete m_windowValidator;
 
-    m_windowValidator = (wxValidator *)validator.Clone();
+    m_windowValidator = static_cast<wxValidator *>(validator.Clone());
 
     if ( m_windowValidator )
-        m_windowValidator->SetWindow(this);
+        m_windowValidator->SetWindow(static_cast<wxWindow*>(this));
 }
 #endif // wxUSE_VALIDATORS