- Ad-Hoc callbacks now receive only a `client` and a `page_data`.
- `page_data` keep all important data useful to process current page in a namedtuple.
- Submitted form is automatically parsed, so callback just has to check the data from there.
- Page index is automatically handled, so the handling by callback is no longed needed.
- Session data is now splitted in 2 dict: private for `AdHocCommandHandler`, and public
for the callback.
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