If a user starts JabRef the second time and does not pass any commands, the existing JabRef instance should be focused.

Implement a new org.jabref.logic.UiCommand to handle the focus. Create and write it correctly.

Hints

1. One other code place is org.jabref.Launcher#handleMultipleAppInstances, where the string This JabRef instance is already running. Please switch to that instance. is logged. --> Modify ´org.jabref.Launcher#handleMultipleAppInstancesto return anenum what to doCONTINUE, SHUTDOWN, FOCUS`.
2. Try out https://docs.oracle.com/javafx/2/api/javafx/stage/Window.html#requestFocus%28%29
3. If it works: implement
4. If it does not work: Close issue as not possible

Internal link: #11783