[REFACTOR] Use the bpmn-visualization API to set element styles

[tbouffard]

Currently, the demo uses custom mxGraph code to style elements.
process-analytics/bpmn-visualization-js#1030 will provide a new way to style the BPMN Elements. So, we should use it here when it is available.

This is currently done at least in:

• bpmn-visualization-pm4py/frontend/src/discovery.js

  Lines 67 to 81 in f75046e

  	let style = mxgraph.mxUtils.setStyle(activityCurrentStyle, 'fillColor', myFrequencyScale(freqValue))
  	mxGraph.getModel().setStyle(activityCell, style);
  	activityCurrentStyle = mxGraph.getModel().getStyle(activityCell)
  	//different ways of setting the style
  	//mxGraph.setCellStyles("fillColor", myFrequencyScale(freqValue), [activityCell]);
  	//or
  	//mxGraph.setCellStyles(mxgraph.mxConstants.STYLE_FILLCOLOR, 'red', [activityCell]);
  	//set label to white when the activity fillColor is above the scale average
  	if (freqValue > avg){
  	style = mxgraph.mxUtils.setStyle(activityCurrentStyle, 'fontColor', 'white')
  	mxGraph.getModel().setStyle(activityCell, style);
  	//different way of setting the style
  	//mxGraph.setCellStyles("fontColor", "white", [activityCell]);
  	}

• bpmn-visualization-pm4py/frontend/src/conformance.js

  Lines 38 to 44 in f75046e

  	mxGraph.getModel().beginUpdate()
  	try {
  	mxgraph.mxUtils.setCellStyles(mxGraph.getModel(), activityCells, "fillColor", "none")
  	mxgraph.mxUtils.setCellStyles(mxGraph.getModel(), activityCells, "fontColor", "none")
  	} finally {
  	mxGraph.getModel().endUpdate();
  	}