Better create-listener syntax
- Use shift and reset instead of nested prompts -- they're equivalent
  and shift/reset has cleaner syntax.

- Clean up event loop code, get rid of unneeded complexity
- Refactor and carefully document event loop code. - 'abort' is only called when necessary, - Error handling simplified and verified
Update documentation for event-loop-prepare!
- Fix bug in for boolean values in valueparams
- Export procedures for accessing values inside of a poll-xcb-connection vector
- Export procedure for getting the fields in an xcb struct