[REFACTOR] Evaluate using "Private class methods" in demo

[tbouffard]

In the past we decided to not use this feature because it was not enough supported in browsers:

• 2023-03-01 https://github.com/process-analytics/bpmn-visualization-examples/pull/467#issuecomment-1449948736
• 2021-09-09 https://github.com/process-analytics/bpmn-visualization-examples/pull/229#pullrequestreview-750523972

Let's review this at the beginning of 2024 and check if it is more largely supported, at least for a long time (at least more than a year).

The "Private class methods" could largely used in demo that rely on the "use case" classes.