Support agent colours

[davidje13]

Extracted from #36

It is already possible to mark agents as red (changing their line colour), but this should be extended to other colours, and it should change the colour of their header/terminator, any state boxes, and possibly any connected notes. This will require some re-working of the rendering / themes code.

begin A
A is red

• [x] red
• [ ] green
• [ ] blue
• [x] agent lines
• [ ] headers and terminators
• [ ] state
• [ ] notes