Adding additional modifiers to be used in bibtex key generator

[matthiasgeiger]

As I currently wanted to have a possibility to have "Capitalized Shorttitles" in my bibtex keys I reused the already existing CapitalizeFormatter, TitleCaseFormatter and SentenceCaseFormatter as "modifiers" to be used also for the BibtexKeyGenerator.

This implements #1506 and is more flexible as the solution developed by @oscargus at the time in #1894 as it can be applied to all other fields and not only to titles.

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• 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• Check documentation status (Issue created for outdated help page at help.jabref.org?) - https://help.jabref.org/en/BibtexKeyPatterns needs to be updated

[tobiasdiez]

Looks good to me. The build error's appear to come from somewhere else (they don't concern the classes changed in this PR).

Please also change the help accordingly after merge.

[Siedlerchr]

If you merge the current master in, the build errors should be resolved. (I accidentally pushed some changes to master yesterday)

[matthiasgeiger]

As build is now passing after rebase I'll merge now.