Closes https://github.com/JabRef/jabref-issue-melting-pot/issues/720

Previously, if your library had a lot of duplicate entries, and you want to quickly merge them, you couldn't do this easily. There is a checkbox "Treat duplicates the same way", but it never worked. This PR solves this program, and it works now like this:

Scenario 1:

1. Running duplicate check
2. Checkbox not checked
3. For each duplicate a merge dialog opens

Scenario 2:

1. Running duplicate check
2. Ticking checkbox for treating duplicates the same way
3. Clicking "Keep merged"
4. Jabref finishes everything else automatically

Scenario 3 (checkbox was ticked previously):

1. Running duplicate check
2. The check box is already ticked
3. Clicking "Keep merged"
4. Jabref finishes everything else automatically

I put Scenario 3 here, because even if the checkbox is ticked, there is a way to untick it.

Steps to test

1. Have a library with a lot of duplicates (for example, run SLR)
2. Run duplicate check
3. Tick "Treat duplicates the same way"
4. Click "Keep merged"

After that JabRef merges duplicates automatically.

Mandatory checks

• I own the copyright of the code submitted and I license it under the MIT license
• I manually tested my changes in running JabRef (always required)
• [/] I added JUnit tests for changes (if applicable)
• [/] I added screenshots in the PR description (if change is visible to the user)
• I described the change in CHANGELOG.md in a way that is understandable for the average user (if change is visible to the user)
• [/] I checked the user 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 updating file(s) in https://github.com/JabRef/user-documentation/tree/main/en.