`WidgetParser` has an unfriendly API

`WidgetParser` takes a `Workspace` in its constructor, which is kind of annoying.  The `Workspace` isn't necessary when you just want to parse an `.nlogo` file for its interface globals and interface global commands.

Also, the interface global commands are returned as a `StringBuilder`, rather than than the more-likely-expected return type, `String`.

It would be semi-nice if these annoyances could be corrected.

I ran into this when implementing f180588c5a66680ea5f0231005871712dc1268d6.