Add support for toplevel ICAL.Todo component for VTODOs

[tobire]

Similar to ICAL.Event, there should be an ICAL.ToDo item that handles the VTODO component as described in section 3.6.2 ("To-Do Component") of RFC 5545 or at https://icalendar.org/iCalendar-RFC-5545/3-6-2-to-do-component.html.

[strukturart]

are there any plans to implement this?

[kewisch]

No immediate plans, but I'd take patches to add it. Most of the consumers I'm working with use this library on the Component/Property level, so there hasn't been an immediate need on my end.