Fix parsing of backslashes at the end of field content

[koppor]

This refs #9668, but does not fully fix it, but eliminates the main issue.

Additional changes:

• New round trip tests at BibEntryWriterTest - they currently work
• Within a class, spell BibTeX consistently.
• ImportHandler should use the provided FileUpdateMonitor
• While working on fixing jabref automatically deletes entries showing wrong syntax #9668, I encountered that the BibTeX entry cannot be pasted using the clipboard. Alsof ixed
  

Tested in the GUI:

This somehow refs JabRef#52.

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• 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 addresses the concrete user description at #9668.

However, there is much work to do address the concrete user request. -- In case an entry cannot be parsed, it is still dropped. See also #9668 (comment)