Tidy up Build tab in Project Properties

[DanTup]

Most stuff on the Build tab isn't currently relevant; I don't think there's value in errors-as-warnings here, as there's not really such thing as a "build" and an important decision with Dart is that warnings don't stop you executing. There is such a thing as a "build", but it's more like a "package for deployment", so I'm not sure if we should tie it to VS's normal build structure (we don't want to kick it off for hitting Ctrl+Shift+B for ex!).

Also see #17, #54, #55.