Add wxWeakRef::element_type, remove unnecessary assignment operator.

Define element_type for consistency with other classes (wxSharedPtr, weak_ptr)
and to allow accessing the type from the template code.

Also remove the apparently unnecessary template assignment operator:
assignment from an expression convertible to type T* works even without it and
while assigning something implicitly convertible to T* wouldn't work now, it's
for the best as we don't want to rely on such implicit conversions.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@63933 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

interface/wx/weakref.h

@@ -102,6 +102,9 @@ template<typename T>
 class wxWeakRef<T> : public wxTrackerNode
 {
 public:
+    /// Type of the element stored by this reference.
+    typedef T element_type;
+
     /**
         Constructor. The weak reference is initialized to @e pobj.
     */
@@ -115,7 +118,7 @@ public:
     /**
         Destructor.
     */
-    ~wxWeakRef();
+    virtual ~wxWeakRef();
 
     /**
         Called when the tracked object is destroyed. Be default sets