Add manual push completion handler support for iOS

Added Display.notifyPushCompletion() to allow applications to manually signal when they have finished handling a push notification on iOS.
This is useful for apps that need to perform background tasks (like playing audio) before the app is suspended.
This feature is enabled by setting the build hint `ios.delayPushCompletion` to `true`.

Author: google-labs-jules[bot]

CodenameOne/src/com/codename1/impl/CodenameOneImplementation.java

@@ -1263,6 +1263,12 @@ public void screenshot(SuccessCallback<Image> callback) {
         callback.onSucess(img);
     }
 
+    /**
+     * Notifies the platform that push notification processing is complete.
+     */
+    public void notifyPushCompletion() {
+    }
+
     /**
      * Returns true if the platform supports a native image cache.  The native image cache
      * is different than just {@link FileSystemStorage#hasCachesDir()}.  A native image cache