[bug] [typescript] Not all publicly exposed and settable props are properly defined

[JamesYeoman]

For instance, the on functions have inline types rather than being specified in interfaces. For example, here's the onSelect definition in the Tree Props interface.

This makes it more difficult, as there aren't any types that I can import to strongly type my functions.

For now, as a workaround, I'll make my own type aliases where necessary.