Add http sever to GUI

[koppor]

For JabMap, we need the http server to interact.

Other refactorings

• Moved shutdown of RemoteListenerManager in HeadlessExecutorService to JabRef shutdown method
• Removed duplicate code

Future work

• configurable port number for the http server: good first issue

Steps to test

1. Start JabRef
2. Enable http server in preferences
3. Open rest-api.http and click on some play buttons (or open Browser http://localhost:23119/libraries/demo)

Mandatory checks

• I own the copyright of the code submitted and I license it under the MIT license
• [/] Change in CHANGELOG.md described in a way that is understandable for the average user (if change is visible to the user)
• [/] Tests created for changes (if applicable)
• Manually tested changed features in running JabRef (always required)
• Screenshots added in PR description (if change is visible to the user)
• [/] Checked developer's documentation: Is the information available and up to date? If not, I outlined it in this pull request.
• [/] Checked documentation: Is the information available and up to date? If not, I created an issue at https://github.com/JabRef/user-documentation/issues or, even better, I submitted a pull request to the documentation repository.

[trag-bot]

@trag-bot didn't find any issues in the code! ✅✨

[calixtus]

As JabMap is dependent on having this integrated and tested, I propose merging this asap, even though there are serious design flaws (HttpServerManager). This needs to be refactored soon. For the other parts, code looks acceptable. 😅