(pylint)

Fix subtle bugs hopefully without introducing new ones

fixes the circular import issue

All this crap is very brittle due to circular deps and python handling
them badly. This appears to be fixing some stuff, at the very least.

This is sed, so cleanup of imports (which can now be grouped together)
is not done yet.

Status and Command are now slotted classes instead of namedtuples,
which led to a few changes to access them with their named parameters
instead of as a tuple.

“short” being a C type, I renamed Command.short into
Command.short_desc, which is more explicit anyway.

I also renamed possible_show into POSSIBLE_SHOW, as it is a
module-level constant dict.

(and correct the display on joined tabs)

It hasn’t worked for more than two years and no one complained.
Furthermore, poezio tries to be smart and request messages based on
previous activity, and not raw numbers.

(thanks eijebong)

e.g. /set . notify_messages false would set the option for a section
named after the current tab.

(the command args parser would never parse three args, making /set
unable to set section-specific options)

/set displays its results in the form “option=value”, so allowing the
command to interpret the same format as a valid command is only logical
(and allows quick copy/paste and edit).

now we can edit stuff, save or cancel those modifications, and change
the chose storage easily

move the modulename to bookmark → boookmarks
add a bookmarklist class with
remove module-level variables
do a features check on startup before trying to fetch
    the bookmarks

Also add /filter_from and /filter_to, and allow chaining filters.