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Also remove the outdated comment saying that there is no base class for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

@@ -15,21 +15,6 @@
 
 #if wxUSE_RADIOBTN
 
-/*
-   There is no wxRadioButtonBase class as wxRadioButton interface is the same
-   as wxCheckBox(Base), but under some platforms wxRadioButton really
-   derives from wxCheckBox and on the others it doesn't.
-
-   The pseudo-declaration of wxRadioButtonBase would look like this:
-
-   class wxRadioButtonBase : public ...
-   {
-   public:
-        virtual void SetValue(bool value);
-        virtual bool GetValue() const;
-   };
- */
-
 #include "wx/control.h"
 
 class WXDLLIMPEXP_FWD_CORE wxRadioButton;
@@ -42,6 +27,8 @@ namespace wxPrivate
     WXDLLIMPEXP_CORE wxRadioButton* wxGetLastButtonInGroup(const wxRadioButton *btn);
 } // namespace wxPrivate
 
+// Unlike most of the other wxXXXBase classes, this one needs to be a template
+// as wxRadioButton derives from different classes in different ports.
 template <class W>
 class wxRadioButtonBase : public W
 {
@@ -50,6 +37,12 @@ public:
 
     wxRadioButtonBase() { }
 
+    // Methods to be implemented by the derived classes:
+    virtual void SetValue(bool value) = 0;
+    virtual bool GetValue() const = 0;
+
+
+    // Methods implemented by this class itself.
     wxRadioButton* GetFirstInGroup() const
     {
         return wxPrivate::wxGetFirstButtonInGroup(static_cast<const wxRadioButton*>(this));