Set Icon for Helper app

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* [x] I have searched the issue tracker for an issue that matches the one I want to file, without success.

**Please describe your issue:**

On MacOS, If you trigger a User Notification from a renderer process (e.g. via a Native Addon), the notification is given a placeholder icon, because the owning .app (the Helper) lacks an icon.


If you specify an Icon, that Icon should also be used for the Helper Apps, or there should be an option to specify the Helper app icon separately.