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Also remove the outdated comment saying that there is no base class for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/msw/radiobut.h

@@ -43,8 +43,8 @@ public:
                 const wxString& name = wxASCII_STR(wxRadioButtonNameStr));
 
     // implement the radio button interface
-    virtual void SetValue(bool value);
-    virtual bool GetValue() const;
+    virtual void SetValue(bool value) wxOVERRIDE;
+    virtual bool GetValue() const wxOVERRIDE;
 
     // implementation only from now on
     virtual bool MSWCommand(WXUINT param, WXWORD id) wxOVERRIDE;