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)