Allow using Bind() with non-public inheritance in C++11 code

Using Bind() with a method of the class deriving from wxEvtHandler
non-publicly used to result in a compile-time error, but at least with C++11
we can detect this case and allow the code to compile.

Closes #17623.

include/wx/event.h

@@ -332,7 +332,7 @@ class wxEventFunctorMethod
               <
                 Class,
                 EventArg,
-                wxConvertibleTo<Class, wxEvtHandler>::value != 0
+                wxIsPubliclyDerived<Class, wxEvtHandler>::value != 0
               >
 {
 private: