bug#52143: 29.0.50; Customize type `group' displays a hanging colon

[Brahimi Saifullah]

Using a `group' widget type inside a `defcustom' form will result in a rather
ugly hanging colon when displayed in the Customize buffer: 

    (defcustom foo '(1 2 3) ""
      :type '(group integer integer integer))

    M-x customize-option foo

After some digging, I found related bugs #31309 and #33566, the latter which
introduced the current `:format' as a workaround to an error.
https://debbugs.gnu.org/cgi/bugreport.cgi?bug=31309
https://debbugs.gnu.org/cgi/bugreport.cgi?bug=33566

>From what I gather: a colon is necessary, for whatever reason.  So having a
tag-less type, as `group' claims to be, is not possible through normal means.

What *is* possible, however, is hiding the colon with a display text property
(The following should go in `group's `define-widget' form, at wid-edit.el):

    :format (concat (propertize ":" 'display "") "%v")

Or with a newline:

    :format (concat (propertize ":" 'display "") "\n%v")

Since the actual text remains unchanged, this should be harmless.  WDYT?

--------------------------------------------------------------------------------

This, of course, is merely polishing the workaround, the problem at its core
remains unchanged.  Though I am not yet privy to the inner working of Customize
and its widgets, all of this seems to stem from some questionable decisions at
`custom-variable-value-create'.

As someone mentioned in one of the linked bug reports that the widget code is
convoluted, I wonder, how well is this code understood? Or perhaps discussing
its behavior is pointless, if nobody can truly understand what the consequences
of righting these questionable decisions would be.