Highlights selected tab in main window

[AugustoMagalhaes]

Issue
Resolves #9152

Approach
Emphasizes the selected tab with a different background color. Side frame buttons can be checked and were grouped into a QButtonGroup to ensure selection uniqueness and also bound the background color to a pseudo selector in their stylesheet.

• 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 unit tests pass locally after every commit (git rebase -i main --exec 'pytest tests/ert/unit_tests -n logical -m "not integration_test"')

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]

Thanks for the contribution!