Enable single backslashes at fileDirectory

[koppor]

My JabRef showed C:git-repositoriesjabrefsrctestresourcespdfs as path. Don't know why. I checked the source and modified the parsing logic that JabRef also accepts paths with single \ in the field.

@Comment{jabref-meta: fileDirectory:C:\git-repositories\jabref\src\test\resources\pdfs;}

When JabRef writes, there will be double backslashes.

This PR also modifes the meta data writing code in a way that the trailing ; of each entry could be removed. I think, this should be done for JabRef 6.0. -- Maybe, also a complete rewrite of the metadata serialization could be done (refs koppor#232)

Minor fixes include

• Rename variable to lower case start
• Use .withField in test

Compulsory checks

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1. Change in CHANGELOG.md described in a way that is understandable for the average user (if applicable)
2. Tests created for changes (if applicable)
3. Manually tested changed features in running JabRef (always required)
4. Screenshots added in PR description (for UI changes)
5. Checked developer's documentation: Is the information available and up to date? If not, I outlined it in this pull request.
6. 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.

[Siedlerchr]

can you add a test case for unc path?

[github-actions]

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