Merge identical group color indicators

[calixtus]

fixes #6175

This PR finishes the PRs of @WangAooa (see #6566 ) and does some cleanup in the MainTableColumnFactory: LinkedIdentifierColumn, FileColumn and SpecialFieldColumn is now extracted in separate classes.

• Change in CHANGELOG.md described (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 documentation: Is the information available and up to date? If not created an issue at https://github.com/JabRef/user-documentation/issues or, even better, submitted a pull request to the documentation repository.

[github-actions]

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WangAooa

[tobiasdiez]

Looks good to me.

[github-actions]

The JabRef maintainers will add the following name to the AUTHORS file. In case you want to use a different one, please comment here and adjust your name in your git configuration for future commits.

Marcel Luethi
WangAooa

[tobiasdiez]

Checkstyle fails. If this is fixed, this can be merged.

[github-actions]

The JabRef maintainers will add the following name to the AUTHORS file. In case you want to use a different one, please comment here and adjust your name in your git configuration for future commits.

Marcel Luethi
Qing
ShiqingLiu
WangAooa

[calixtus]

Checkstyle is happy again, so I am merging.