[FEAT] Provide a way to configure icon stroke and fill color

[tbouffard]

Is your feature request related to a problem? Please describe. Today, the color of the icon provided by the lib are based on the fill and stroke color of their enclosing shape. This is not possible to specify them by setting a mxGraph style attribute. For now, user are forced to provide a custom IconPainter implementation like in https://github.com/process-analytics/bpmn-visualization-examples/pull/63

Describe the solution you'd like Consider new mxGraph style attributes dedicated to the fill and stroke icon/marker colors. If not specified, let’s take what is defined for the englobing shape (as today) The style should be set in the paintParameter and the icon paint functions should use this style information Also review the way we currently fill icon: we sometimes use stroke color as fill color (for instance for throwing event icon) in the painting code. In that case, the fill color should probably be set to the stroke color in the paintParameter and in the icon painting code, we always use this if we want to consider the style configuration.

Additional context For an additional way to extend colors configuration, see #424