I have a problem with FunFormKit 0.4.1. If I check some values in a
`MultiCheckboxField` display and the form is redisplayed after a
failed validation, the check marks are removed in the redisplayed
form. For comparison, `TextField`s and `RadioField`s work fine.
Another problem I found in code for some fields; in several places the
format `"%s_%2i" % (name, id)` is used for ids. However, for ids lower
than 10 this results in names like "name_ 3" (note the space) which
may cause trouble with some browsers.
Additionally, I had a misunderstanding regarding the docs (or the
source code): For mutable form definitions, I used `fieldNames` to
request and then delete all fields (at least, I thought that would
work ;-) ). However, a few fields were/are not in the list returned by
`fieldNames`; I had to use `fields().keys()` instead to get all