merge entries should be able to merge keywords

[YANG-CY-163]

Fixes JabRef#12

• 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]

@YANG-CY-163 Thank you for starting working on this.

Please investigate following issues:

• The merging of keywords is some kind of logic. That should not happen in the UI. Please create a method static KeywordList merge(KeywordList other) {} (in org.jabref.model.entry.KeywordList) returning a new KeywordList
• There need be test cases for merging keywords. Create some test methods in org.jabref.model.entry.KeywordListTest

Since another contributor works on the UI, you can stop implementing something in the UI. Please open a new branch addKeywordMerge from JabRef:main and just implement the above methods. Then, open a new PR. Then, close this PR.

Nevertheless, please add a screenshot of your new dialog so the community can benefit from your idea.

[Siedlerchr]

Superseded by #9022