[REFACTOR] Use a single BPMN container in the "monitoring" demo

[tbouffard]

This is the same request as in #526 and #540.
We use several HTML container whereas we could use a single one to demonstrate interactivity on the same BPMN diagram.

Implementing it will be easier the following features to be available:

• reset the style of BPMN elements: [FEAT] Add a new API to reset the modified style of given elements via APIs bpmn-visualization-js#2458
• remove all overlays: [FEAT] Let remove all overlays of several BPMN elements at once bpmn-visualization-js#2577

This should be possible with more code with the existing API.