Handle coloring of reaction parts

CellDesigner allows coloring of reaction parts (see below), these should be handled.

added CellDesigner label

Here is example showing different colors used in reactants, products and modifiers coloring.xml

added SBML graphics; css; layout labels

created branch 597-handle-coloring-of-reaction-parts

mentioned in merge request !706 (merged)

closed via merge request !706 (merged)

mentioned in commit c7b5c331