Add support for XML coloring in the renderer

[jan-ladleif]

Fixes #152, fixes #169

Basically the renderer now natively supports colors as defined in the diagram exchange information in the XML.
Since the standard is relatiely vague on how coloring should work, I made several decisions:

• Since messages do not have their own display information in the XML, they are rendered according to the participant band they are attached to.
• If a choreography activity has a custrom stroke color, all participant band dividers and markers are colored in that color as well.
• If a participant band has no specific color, it somewhat blends in with the choreography activity it is attached to.
• If multiple participant bands on sub-choreographies have different stroke colors, those might overlap.
• Call activities have some not-so-nice artifacts if a participant band has a stroke.
• ... probably a bit more, can be discussed!

Example diagram: example.txt (rename to .bpmn)
From top to bottom:

1. Colored participant bands
2. Colored activities
3. Colored participant bands and activities
   

[jan-ladleif]

Still need to test the bpmn-js API: https://github.com/bpmn-io/bpmn-js-examples/tree/master/colors#option-2-color-via-bpmn-20-extension

[jan-ladleif]

The color change command

modeling.setColor(event.element, {
  stroke: 'green',
  fill: 'yellow'
});

should now also work.
Again, there needs to be special code for participant bands and messages since they do not have a regular di reference.