Explicitly specify application class in wxiOS

Although @"UIApplication" is supposed to be used by default anyhow,
passing "nil" is reported to result in an assertion failure in
'_UIApplicationGetPrincipalClass' when running under the iOS 13.1
simulator, so pass this string explicitly.

Closes https://github.com/wxWidgets/wxWidgets/pull/2035

src/osx/iphone/evtloop.mm

@@ -90,7 +90,7 @@ void wxGUIEventLoop::OSXDoRun()
     {
         wxMacAutoreleasePool pool;
         const char* appname = "app";
-        UIApplicationMain( 1, (char**) &appname, nil, @"wxAppDelegate" );
+        UIApplicationMain( 1, (char**) &appname, @"UIApplication", @"wxAppDelegate" );
     }
     else 
     {