this is in preparation for adding more separate "modes" to the tui, in
particular, a "transcript mode" to view a full history once #2316 lands.
1. split apart "tui events" from "app events".
2. remove onboarding-related events from AppEvent.
3. move several general drawing tools out of App and into a new Tui
class
Wait for newlines, then render markdown on a line by line basis. Word wrap it for the current terminal size and then spit it out line by line into the UI. Also adds tests and fixes some UI regressions.
