bug#7561: Soft hyphens should probably normally not be shown

[Stefan Monnier]

> The soft hyphen (173) is currently shown in the buffer. That is
> unexpected to me though it seems to be a "hard problem":

>   Soft hyphen (SHY) - a hard problem?
>   http://www.cs.tut.fi/~jkorpela/shy.html

> I would expect it to be shown only if a line break happens within a word.

Regardless of the controversies over its actual semantics, Emacs
historically has tended to display all chars (control or not), because
it is meant to give you access to the markup rather than its rendering.

Admittedly, over time, Emacs has added various wysiwyg-ish features, but
it is still the case that completely hiding a char, by default, is
considered as undesirable since it makes it more difficult to manipulate
it (delete/insert/move/...).

So if/when we decide to hide such a char, it will be under the control
of something like a variable which a major-mode can set to indicate that
this buffer is meant as the rendering of a piece of text, rather than
meant for editing that text.


        Stefan