Hi all,

A rough version of the patch we were talking about in the other thread
on persistent naming. To recap:

New defcustoms, tab-bar-post-open-functions and
tab-bar-pre-close-functions. Call all of the function in
post-open-functions at the end of tab-bar-new-tab-to, passing in the new
tab as an argument. Similar for pre-close-functions, but slightly more
complicated to lend some expressive power. Each of the functions take 2
arguments, the tab itself, and a flag indicating whether or not it's the
last tab. The big thing is that the return values are kept track of, and
if *any* of the functions return a non-nil value
(ie `(cl-some (mapcar (lambda (x) (funcall x tab last-tab))) pre-close-functions)`
is non-nil), it short-circuits and doesn't run the rest of
tab-bar-close-tab. This would be useful to provide to users so
that they can do other clean-up tasks when the tab is closed, as well as
providing a way to protect a tab from being closed. Ditto for being able
to do some tab set-up when you open a new tab.

This is going to take some additional work. I couldn't think of a clean
way to gracefully provide a way to easily make permanent changes to the
tab for the purposes of tab-bar-post-open-functions - the functions
within have to grovel through the tabs frame parameter and write back a value
themselves. I personally don't find this an optimal user experience, but
feedback on that point is welcome.

This will also need to be documented in the manual, but I can do that
after we hash out some details - I just didn't want to make changes to
the manual until we're satisfied with the semantics of this whole thing.

Patch follows. Thoughts on it appreciated.

Thanks,
-- 
~Robert Cochran

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