bug#12872: 24.2; Provide a feature to trigger mode-line redisplay

[Eli Zaretskii]

> From: "Drew Adams" <drew.adams@oracle.com>
> Cc: <12872@debbugs.gnu.org>
> Date: Mon, 12 Nov 2012 14:17:38 -0800
> 
> I wonder (without being very familiar with the mode-line %-constructs, so 
> maybe
> speaking nonsense) whether it might be useful to add specific %-constructs (or
> variables?) that say when (i.e., in what contexts) to trigger mode-line
> redisplay.

What would be the advantages of such a feature?  Since the mode line
format is not even consulted in order to decide whether or not to
redisplay the mode line (because its processing is very non-trivial,
what with all the propertize stuff and Lisp expressions going on there
even in the standard value), we will need internal variables to shadow
these constructs anyway.

Having variables or maybe a plist of the mode line format allows
easier access to this information, and is IMO more Lispy than hiding
the information in some % magic.