[wip] 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.

Wip.

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?
• ...

Goal:

• Level of abstraction? (LibraryTab in StateManager instead of BibDatabaseContext)
• 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.