Allow constructing/assigning wxObjectDataPtr from compatible type

Generalize copy ctor and assignment operators to allow implicit
conversions from wxObjectDataPtr<D> to wxObjectDataPtr<B> if D is
implicitly convertible to B (e.g. if B is the base class and D is a
class derived from it).

This makes wxObjectDataPtr<> more like standard smart pointer classes
and more useful.

include/wx/object.h

@@ -278,6 +278,15 @@ public:
             m_ptr->IncRef();
     }
 
+    // generalized copy ctor: U must be convertible to T
+    template <typename U>
+    wxObjectDataPtr(const wxObjectDataPtr<U> &tocopy)
+        : m_ptr(tocopy.get())
+    {
+        if (m_ptr)
+            m_ptr->IncRef();
+    }
+
     ~wxObjectDataPtr()
     {
         if (m_ptr)
@@ -330,6 +339,17 @@ public:
         return *this;
     }
 
+    template <typename U>
+    wxObjectDataPtr& operator=(const wxObjectDataPtr<U> &tocopy)
+    {
+        if (m_ptr)
+            m_ptr->DecRef();
+        m_ptr = tocopy.get();
+        if (m_ptr)
+            m_ptr->IncRef();
+        return *this;
+    }
+
     wxObjectDataPtr& operator=(T *ptr)
     {
         if (m_ptr)