Replaces the standard ComboBox with a SearchableComboBox and a free text field in custom Entry Types

[Memeel]

Closes #14082

This pull request replaces the standard ComboBox for adding new fields with a SearchableComboBox and a TextField in the "Custom Entry Types" preference pane.

This improves usability by allowing users to quickly filter and find a specific field when customizing entry types. The option is located under File -> Preferences -> Entry types.

Steps to test the SearchableComboBox

1. Go to File -> Preferences -> Entry types.
2. Select any entry type from the list on the left.
3. In the "Required and optional fields" section, look at the first dropdown menu at the bottom.
4. Verify that this is now a SearchableComboBox:
   • Click on the dropdown. It should show the full list of available fields (first screenshot).
   • Start typing the name of a field (second screenshot).
   • Verify that the list filters dynamically to show matching fields.
5. Select a field from the filtered list and click the + button.
6. Verify that the field is added correctly to the list of fields for that entry type.

Steps to test the free text field

1. Go to File -> Preferences -> Entry types.
2. Select any entry type from the list on the left.
3. In the "Required and optional fields" section, look at the second input box (first screenshot).
4. Type a new custom field name and click the + button (second screenshot).
5. Verify that the field is added correctly to the list of fields for that entry type.

Final result

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.

[github-actions]

Hey @Memeel!

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[Siedlerchr]

Free text field is missing

[calixtus]

No changes to project files

[Memeel]

You're right. Sorry, I accidentally removed a line in a previous commit, and the latest one reintroduces it

[Memeel]

Just to make sure we are not misunderstanding the task, are we supposed to add another box below the existing one that lets you add the free text field, or should we keep a single combobox that has the searchable feature and the adding free text field?

[Siedlerchr]

As a User I want to search for existing fields and also have the option to create a new, not yet existing one, e.g. "mycustomfield" . If this is possible with the SearchableCombobox then it's good, otherwise add a new text field where the user can enter the fieldname