v2: TUI UI – "higher level" abstractions

[robbielyman]

• [x] currently seamstress emits render events periodically. render events should instead be requested by Lua.
  • [x] something like seamstress.update.time = 1/60 should request that seamstress emit an update event at 60Hz
    • 60Hz meaning rain or shine, as in attempting to stay on beat even if in actuality the event fires a little late
    • making the value 0 or negative cancels the timer
    • if any callback returns _, true from update, a { 'tui', 'draw' } event is emitted
    • eventually the same should be true for GUI window draw events
• [x] tui event responders should trigger { 'tui', 'draw' } if any of them return _, true
  • [x] i think this has to be hardcoded (i.e. in Zig?) because that is who is calling publish?
• [ ] there should be a higher-level set of UI primitives built in to seamstress.tui
  • [ ] Panes contain other primitives and provide relative coordinates
  • [ ] Buttons respond to click events by calling a function
  • [ ] Tabs stack Panes
  • scrollable text boxes? how custom should the REPL be?