Avoid unrealizing a frozen window

It seems to continue to prevent updates to the affected area

Fixes #13543


git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@72569 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/gtk/window.h

@@ -204,6 +204,7 @@ public:
     // Called when m_widget becomes realized. Derived classes must call the
     // base class method if they override it.
     virtual void GTKHandleRealized();
+    void GTKHandleUnrealize();
 
 protected:
     // for controls composed of multiple GTK widgets, return true to eliminate