Improve support for todo widgets

Initial work in https://github.com/zulip/zulip-terminal/pull/1549 by @rsashank provided

• parsing of todo data
• rendering into a styled message
• updating of the styled message upon changes in the server (events)

This issue is intended to split out todo-related follow-up work, part of which was originally tracked in #986.

Outstanding elements include

• Enable responding to tasks, ie. toggling the 'check mark' or 'done' status
• Allow editing the title in your own todo list
• Allow adding tasks to todo lists
• Limiting the editing of todo lists in the client (widget messages (content) may not be edited)