Notify WebView2 API when minimizing a window with wxWebView

As suggested by the ICoreWebView2Controller documentation
call put_isVisible() when the window containing the
wxWebViewEdge is minimized/restored.

include/wx/msw/webview_edge.h

@@ -105,6 +105,8 @@ private:
 
     void OnSize(wxSizeEvent& event);
 
+    void OnTopLevelParentIconized(wxIconizeEvent& event);
+
     bool RunScriptSync(const wxString& javascript, wxString* output = NULL) const;
 
     wxDECLARE_DYNAMIC_CLASS(wxWebViewEdge);