Don't destroy the native window in wxNativeWindow itself by default

Leave ownership of the native window to the user code as it may want to reuse
it for some other purpose and provide an explicit Disown() function that can
be called if the user really wants wxWidgets to take ownership of the native
window.

In particular, this avoids problems when using ARC under OS X which resulted
in a double "release" before.

include/wx/nativewin.h

@@ -69,7 +69,10 @@ class WXDLLIMPEXP_CORE wxNativeWindow : public wxWindow
 {
 public:
     // Default ctor, Create() must be called later to really create the window.
-    wxNativeWindow() { }
+    wxNativeWindow()
+    {
+        Init();
+    }
 
     // Create a window from an existing native window handle.
     //
@@ -80,13 +83,46 @@ public:
     // 0 if the handle was invalid.
     wxNativeWindow(wxWindow* parent, wxWindowID winid, wxNativeWindowHandle handle)
     {
+        Init();
+
         Create(parent, winid, handle);
     }
 
     // Same as non-default ctor, but with a return code.
     bool Create(wxWindow* parent, wxWindowID winid, wxNativeWindowHandle handle);
 
+    // By default the native window with which this wxWindow is associated is
+    // owned by the user code and needs to be destroyed by it in a platform
+    // specific way, however this function can be called to let wxNativeWindow
+    // dtor take care of destroying the native window instead of having to do
+    // it from the user code.
+    void Disown()
+    {
+        wxCHECK_RET( m_ownedByUser, wxS("Can't disown more than once") );
+
+        m_ownedByUser = false;
+
+        DoDisown();
+    }
+
+#ifdef __WXMSW__
+    // Prevent the native window, not owned by us, from being destroyed by the
+    // base class dtor, unless Disown() had been called.
+    virtual ~wxNativeWindow();
+#endif // __WXMSW__
+
 private:
+    void Init()
+    {
+        m_ownedByUser = true;
+    }
+
+    // This is implemented in platform-specific code.
+    void DoDisown();
+
+    // If the native widget owned by the user code.
+    bool m_ownedByUser;
+
     wxDECLARE_NO_COPY_CLASS(wxNativeWindow);
 };