[REFACTOR] Improve the "Conformance Data" code

[tbouffard]

Notes: do not hesitate to split this issue to better track changes progress

modifier le fill via le model state/mxgraph (mettre lien vers issue style)

• [ ] Update the fill color of element with gradient by using the mxgraph state or wait for https://github.com/process-analytics/bpmn-visualization-js/issues/1030
• [ ] Rework the linear gradients creation. We should use the mxgraph graph.canvas property, put the initialization code in a function, and check how mxGraph create/delete his own gradient for inspiration
• [ ] Simplify the elements creation code. It is very complicated, the CSS selectors are not precise enough and some are redundant