bug#27544: 25.1; Visualization of Unicode bidirectional marks

[Juri Linkov]

>> > Thanks.  But wouldn't glyphless-hex-display-mode be a better name?
>> > glyphless-display-mode doesn't really say what it does to glyphless
>> > characters.
>>
>> It doesn't hexify the glyphs, it uses `acronym'.  But I thought we might
>> allow that to be customised if somebody asks for it, which is why it's
>> not called `glyphless-acronym-display-mode'.
>
> But glyphless-display-mode has no mnemonic value.  We already display
> glyphless characters, so this name doesn't help at all.
>
> Does anyone have suggestions for a better name?

"glyphless-display-mode" looks fine - I read it as to mean
a mode that defines how to display glyphless chars.
It could also provide a defcustom with a choice from
glyphless-char-display-method.

Where I see the problem is that the whole new file was created in
lisp/textmodes/glyphless-mode.el for just glyphless-display-mode,
instead of adding it to lisp/international/characters.el
where this feature is defined with all its functions
glyphless-char-display-control, update-glyphless-char-display,
and more.