Make AiService global

[InAnYan]

Closes #7.

I made the AiSevice live inside
JabRefGUI. It lives alongside the PreferenceService, and other classes that are often referenced in constructors.

Yes, I can't find the appropriate word for "classes that are often used in constructors", so I hope the reviewers will understand what I mean by looking at the changes (yes, I understand that's not the right way to behave).

Mandatory checks

• Change in CHANGELOG.md described in a way that is understandable for the average user (if applicable)
• Tests created for changes (if applicable)
• Manually tested changed features in running JabRef (always required)
• Screenshots added in PR description (for UI changes)
• 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.

[koppor]

This is kind of the "old" way - superseded by JabRef#11342, but 11342 will be part of the next release... so, let's move on with this.

[InAnYan]

Okay, I checked the files again and came to conclusion:

• Where you used a private final field and initialized it in constructor, I did the same for AiService.
• Where you used an @Inject private final, I used it.

[koppor]

@InAnYan 🤣🤣 - I think, you found out what @Contracts are offered by JabRef (these can be injecgted; the others cannot) - See koppor#687 (comment) for some more links.

I created an internal issue (https://github.com/JabRef/jabref-issue-melting-pot/issues/440) to label all "contracts" as such. - If you have capacity, you can do it using your experienced gained. IMHO, these are 3 to 5 classes being contracts or are there more?

[koppor]

Other thing: Delete the merged branches. You can do it after merging, have refined github browser plugin doing it automatically, or use the branch overview now and then to delete.