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Also remove the outdated comment saying that there is no base class for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

@@ -46,6 +46,10 @@ public:
                 const wxValidator& validator = wxDefaultValidator,
                 const wxString& name = wxASCII_STR(wxRadioButtonNameStr));
 
+    // (re)implement pure virtuals from wxRadioButtonBase
+    virtual void SetValue(bool value) wxOVERRIDE { return wxCheckBox::SetValue(value); }
+    virtual bool GetValue() const wxOVERRIDE { return wxCheckBox::GetValue(); }
+
     // override some base class methods
     virtual void ChangeValue(bool value) wxOVERRIDE;