[FEAT] Set the overlays default colors using the hexa form #73

Is your feature request related to a problem? Please describe.
The default overlays colors in v0.1.1 are defined in the rgba form. Please use hexa color form.

Describe the solution you'd like
We generally use the hexa forms in the lib examples.
The rgba currently used in the js bridge code is not well supported in Inkscape: this was an issue when we exported the SVG generated in the R View and edit it in this software. See #72.

Additional context
See https://github.com/process-analytics/bpmn-visualization-R/blob/v0.1.1/inst/htmlwidgets/bpmnVisualization.js#L42.