Extract Entry Editor

[calixtus]

Instead of having an instance of the EntryEditor for every LibraryTab - one is enough. One EntryEditor to rule them all, one EntryEditor to find them, one Entryeditor to bring them all and in the bottom of the interface bind them.

Introduced

• Interface PreviewControls

Todo:

• Move Open/Close of EntryEditor to JabRefFrame
• Adapt EntryEditor to follow observables and to tolerate no entry
• Adapt StateManager from DatabaseContext to LibraryTab? (invalid)
• Fix remaining bugs:
  • Not showing anything
  • NPE on exit
  • Open entry editor menu item
  • Entry editor maximized when open

Goal:

• Level of abstraction (StateManager should use LibraryTab instead of BibDatabaseContext)
  postponed to later pr. would overload this one
• Easy rewrite of entry editor in the future
• Unwind dependencies

Mandatory checks

• I own the copyright of the code submitted and I licence 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 (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]

Related historic PRs:

• Avoid recreation of the EntryEditor #3187 ("Avoid recreation of the EntryEditor")
• Improve performance of entry editor #2892 ("Improve performance of entry editor")

Side note: I found the PR for the maintable JavaFX migration: #3621 (and #3591)

[subhramit]

There can only be one.

[subhramit]

Works fine for me. Although I noticed some exception logs (not related to these changes) appearing when I double click on an entry to bring up the editor after startup.
Filed an issue at #12494

[github-actions]

The build of this PR is available at https://builds.jabref.org/pull/12210/merge.