Add wxRadioButtonBase::{Set,Get}Value()

Now that we do have wxRadioButtonBase class, declare wxRadioButton API
methods as pure virtual in it, to force the derived classes to implement
them.

Also remove the outdated comment saying that there is no base class for
wxRadioButtons in different ports, this is not true any longer.

include/wx/gtk/radiobut.h

@@ -39,8 +39,8 @@ public:
                  const wxString& name = wxASCII_STR(wxRadioButtonNameStr) );
 
     virtual void SetLabel(const wxString& label) wxOVERRIDE;
-    virtual void SetValue(bool val);
-    virtual bool GetValue() const;
+    virtual void SetValue(bool val) wxOVERRIDE;
+    virtual bool GetValue() const wxOVERRIDE;
 
     static wxVisualAttributes
     GetClassDefaultAttributes(wxWindowVariant variant = wxWINDOW_VARIANT_NORMAL);