Fix best size for windows which are hidden when TLW is shown with GTK3

GTK3's style cache is not updated for hidden windows until after they are shown
See #16088

include/wx/gtk/window.h

@@ -493,6 +493,7 @@ public:
         return m_paintContext;
     }
     void GTKSizeRevalidate();
+    void GTKSendSizeEventIfNeeded();
 #endif
 
     wxDECLARE_DYNAMIC_CLASS(wxWindowGTK);