Manually call Cef.Shutdown to avoid hangs

CefSharp can hang if exit is done via Environment.Exit.

cefsharp/CefSharp#990

Manually call shutdown.

src/ServiceStackVS/ProjectTemplates/ReactJSGap/ReactJSWinForms/NativeHost.cs

@@ -96,6 +96,7 @@ public void PerformUpdate()
                 formMain.InvokeOnUiThreadIfRequired(() =>
                 {
                     formMain.Close();
+                    Cef.Shutdown();
                     UpdateManager.RestartApp();
                 });
             });