This is compatible with Javascript, and it also seems like a better
default; when people forget to think about return values in click
handlers, they should be consumed.
Symptom: a test (test_click_to_create_drawing) started randomly failing
after I inserted a `return` 2 commits ago.
Cause: my tests call edit.draw, but button handlers only get cleared in
app.draw. So my tests weren't clearing button handlers, and every call
to edit.draw was accumulating states. Still unclear why those were going
to different state objects after the `return`, but anyway. I'm not going
to understand every last thing that happens when things go wrong, just
guarantee they can't go wrong. And the way to do that is to decentralize
button handlers to each state that receives them.
The State object in buttons.lua doesn't have to be Editor_state. It just
has to be some table that provides a Schelling Point for shared state.
Kartik K. Agaram