Linup API of wxRadioBox::Show on all ports. Move wxRadioBox::IsValid from wxUniversal to base class (+ GTK which do not use base class) and use it where applicable. Minor source cleaning.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@32325 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/ctrlsub.h

@@ -65,6 +65,11 @@ public:
     // reads better for multi-selection ones
     void Select(int n) { SetSelection(n); }
 
+
+protected:
+
+    // check that the index is valid
+    inline bool IsValid(int n) const { return n >= 0 && n < GetCount(); }
 };
 
 class WXDLLEXPORT wxItemContainer : public wxItemContainerImmutable