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.

On xmltab and listtab, the help text would be displayed in the input
after closing the tab.

fix #3192

e.g. if there is nothing to reset

if the error has the same id as a sent message, it will be displayed
    with a cross where there is usually a checkmark (ack), and the
    received error will be appended to the message, in red.
if it does not have a know id, it will be added as another message to
    the conversation, without a nick, and in red.

(also make the message appear in PrivateTabs)

Avoid surprises with some commands accepting quoted arguments and some other
not.

fix #2555

it’s confusing.

Make the "default" parameter optional and thus move it to the end of
the command with the other optional parameters.

And change all the calls.

Improvements: events now occur precisely at the specified date. You don’t
need to stop touching your keyboard to execute them.

also, check the features every rejoin for PrivateTabs
and refresh the window after displaying the features

and add a handler for missing commands

no more stalling while waiting for a disco info while sending a message.

This reverts commit b46f0f5e.

Conflicts:
	src/tabs/muclisttab.py

Doing this made the unresized elements refresh in the old subwins,
causing glitches and weirdness. And anyway, the only problematic
element is the TextWin (rebuilding all the lines of a buffer is
expensive), but it already checks if the width changed.

(keep a get_name() fallback just in case for now)

- also, fix the /xhtml command that was nesting one <body/> too many