Change look&feel of simulation mode combobox

[andreas-el]

Checked on RGS, looks as intended there too. ✅

Was:

Change:

• PR title captures the intent of the changes, and is fitting for release notes.
• Added appropriate release note label
• Commit history is consistent and clean, in line with the contribution guidelines.
• Make sure tests pass locally (after every commit!)

When applicable

• When there are user facing changes: Updated documentation
• New behavior or changes to existing untested code: Ensured that unit tests are added (See Ground Rules).
• Large PR: Prepare changes in small commits for more convenient review
• Bug fix: Add regression test for the bug
• Bug fix: Create Backport PR to latest release

[eivindjahren]

👍

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 42.85714% with 4 lines in your changes missing coverage. Please review.

Project coverage is 90.67%. Comparing base (83ec3c4) to head (a9bbea8).
Report is 1 commits behind head on main.

Files	Patch %	Lines
...rc/ert/gui/ertwidgets/combobox_with_description.py	42.85%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #8461      +/-   ##
==========================================
- Coverage   90.68%   90.67%   -0.02%     
==========================================
  Files         346      346              
  Lines       21013    21018       +5     
==========================================
+ Hits        19056    19058       +2     
- Misses       1957     1960       +3     

Flag	Coverage Δ
gui-tests	75.92% <42.85%> (+0.02%)	⬆️
integration-tests	53.98% <42.85%> (-0.01%)	⬇️
unit-tests	68.71% <42.85%> (-0.01%)	⬇️

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