Override DoEnable() instead of Enable() in wxGTK controls

This is slightly simpler, as it doesn't require checking whether the
control state really changes or not (it always does if DoEnable() is
called) and allows disabling the controls before creating them, e.g.
code like

    wxButton* const b = new wxButton();
    b->Disable();
    b->Create(this, wxID_OK);

works as expected now instead of spewing GTK+ errors.

src/gtk/radiobox.cpp

@@ -487,8 +487,18 @@ void wxRadioBox::SetString(unsigned int item, const wxString& label)
 
 bool wxRadioBox::Enable( bool enable )
 {
-    if ( !wxControl::Enable( enable ) )
-        return false;
+    // Explicitly forward to the base class just because we need to override
+    // this function to prevent it from being hidden by Enable(int, bool)
+    // overload.
+    return wxControl::Enable(enable);
+}
+
+void wxRadioBox::DoEnable(bool enable)
+{
+    if ( !m_widget )
+        return;
+
+    wxControl::DoEnable(enable);
 
     wxRadioBoxButtonsInfoList::compatibility_iterator node = m_buttonsInfo.GetFirst();
     while (node)
@@ -503,8 +513,6 @@ bool wxRadioBox::Enable( bool enable )
 
     if (enable)
         GTKFixSensitivity();
-
-    return true;
 }
 
 bool wxRadioBox::Enable(unsigned int item, bool enable)