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/weakref.h

@@ -19,7 +19,7 @@
 template <class T>
 struct wxIsStaticTrackable
 {
-    enum { value = wxConvertibleTo<T, wxTrackable>::value };
+    enum { value = wxIsPubliclyDerived<T, wxTrackable>::value };
 };