push WidgetParser down to workspace package

Tortoise needs it, and we want to reduce Tortoise's dependence on very
high-level packages like headless

partially addresses #454
make WidgetParser.parseWidgets return more sensible types

in particular, avoid returning StringBuilder. yuck

partially addresses #454
Merge pull request #454 from NetLogo/widgetparser-cleanup

`WidgetParser` has an unfriendly API