Move m_labelWin to wxStaticBoxBase itself

It will be reused by all platforms and is not specific to wxGTK.

This also means WXDestroyWithoutChildren() doesn't need to be virtual
any longer.

include/wx/statbox.h

@@ -49,12 +49,16 @@ public:
     //
     // Reparent all children of the static box under its parent and destroy the
     // box itself.
-    virtual void WXDestroyWithoutChildren();
+    void WXDestroyWithoutChildren();
 
 protected:
     // choose the default border for this window
     virtual wxBorder GetDefaultBorder() const wxOVERRIDE { return wxBORDER_NONE; }
 
+    // If non-null, the window used as our label. This window is owned by the
+    // static box and will be deleted when it is.
+    wxWindow* m_labelWin;
+
     wxDECLARE_NO_COPY_CLASS(wxStaticBoxBase);
 };