Rework external changes dialog in JavaFX

[tobiasdiez]

Convert "External changes" dialog to JavaFX.

Notification is now at the top of the library instead of in the sidepane:

Converted dialog:

---

• Change in CHANGELOG.md described
• Tests created for changes
• Manually tested changed features in running JabRef
• Screenshots added in PR description (for bigger UI changes)
• Ensured that the git commit message is a good one
• Check documentation status (Issue created for outdated help page at help.jabref.org?)

[Siedlerchr]

I can't get it work properly.
Open a JabRf bib file.
Modify the bib file externally in Notepad++
Save the change sin Notepad++
JabRef does not show anything.

I debugged and set a breakpoint;
And I could capture an
Not on FX application thread; currentThread = pool-3-thread-5

 public EntryChangeViewModel(BibEntry memEntry, BibEntry tmpEntry, BibEntry diskEntry) {

         fieldChanges.add(new FieldChangeViewModel(field, memEntry, tmpEntry, mem.orElse(null), tmp.orElse(null),   disk.orElse(null))); //this line causes the exception

The problematic class is this: The WebView must be initialized on FX Thread

 static class FieldChangeViewModel extends DatabaseChangeViewModel {

        private final BibEntry entry;
        private final BibEntry tmpEntry;
        private final String field;
        private final String inMem;
        private final String onDisk;
        private final WebView tp = new WebView();

[Siedlerchr]

And the GUI is unreadable has hell:

And there is also a problem when doing multiple changes before I click on any review:

[Siedlerchr]

see screenshots

[tobiasdiez]

God damn, was I too lazy to provide a properly working interface. Now everything should be fine (will still not win a prize for best ui).