Fix using WX_DECLARE_OBJARRAY() with pointer types

While this doesn't make, and never made, any sense (pointers should be
stored in a plain, non-object array), we still should continue to
support it for compatibility.

Fix the problem introduced by 97684a9267
by writing "T const&" instead of "const T&" as the latter didn't have
the same meaning when the macro parameter "T" expanded into "U*" as
"const" applied to "U", making this type incompatible with the actual
one.

Also extend the unit test to check for this.

include/wx/dynarray.h

@@ -652,7 +652,7 @@ private:
     classdecl wxObjectArrayTraitsFor##name                                    \
     {                                                                         \
     public:                                                                   \
-        static T* Clone(const T& item);                                       \
+        static T* Clone(T const& item);                                       \
         static void Free(T* p);                                               \
     };                                                                        \
     typedef wxBaseObjectArray<T, wxObjectArrayTraitsFor##name>                \