feat: Add extra slots to Split Panel header

[jperals]

Description

Related links, issue #, if available: n/a

How has this been tested?

Review checklist

The following items are to be evaluated by the author(s) and the reviewer(s).

Correctness

• Changes include appropriate documentation updates.
• Changes are backward-compatible if not indicated, see CONTRIBUTING.md.
• Changes do not include unsupported browser features, see CONTRIBUTING.md.
• Changes were manually tested for accessibility, see accessibility guidelines.

Security

• If the code handles URLs: all URLs are validated through the checkSafeUrl function.

Testing

• Changes are covered with new/existing unit tests?
• Changes are covered with new/existing integration tests?

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.00%. Comparing base (38348b2) to head (a1bbb7f).

Additional details and impacted files

@@           Coverage Diff            @@
##             main    #3740    +/-   ##
========================================
  Coverage   97.00%   97.00%            
========================================
  Files         821      821            
  Lines       23862    23883    +21     
  Branches     8306     7966   -340     
========================================
+ Hits        23147    23168    +21     
- Misses        666      709    +43     
+ Partials       49        6    -43     

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