Add extra description field to each freight car

[GoogleCodeExporter]

Some modelers use car descriptions rather than reporting marks to make it 
easier to find cars in yards:

"SP 20394 RED"
"BOX SP YELLOW"

We ought to figure out a way to include car descriptions in switchlists, 
perhaps by having the car description on a separate line of each switchlist.

If it's not easy, we ought to include helpful workarounds in the documentation.

Complications:
* Need to redo all switchlist styles to fit the car descriptions
* Makes switchlists longer

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Original issue reported on code.google.com by rwbowdi...@gmail.com on 21 Aug 2011 at 6:06