redo version checks yet again

redo version checks yet again

I'm starting to feel better after replacing 1 line with 20 and 2 new
bits of global state. I'm now handling two scenarios more explicitly:

* If I change Current_app within key_press, the corresponding text_input
  and key_release events go to the new app. If it's an editor it might
  insert the key, which is undesirable. Putting such handlers in
  key_release now feels overly clever, particularly since it took me
  forever to realize why I was getting stuck in an infinite loop.

* Both 'run' and 'source' can hit the version check, so we need to be
  able to transition from the 'error' app to either. Which
  necessitates yet another global bit of state: Next_app.