bug#7851: `customize's presentation of :type (repeat plist) declarations

[MON KEY <monkey@sandpframing.com>]

`customize's presentation of variables defined with a type speciifer:

 :type '(repeat plist :value-type (repeat symbol))

do not present a sensibly in customize buffers.

For example:

(defcustom *tt--ugly-plist-custom*
  '((:KEY-PL0-0 (sym0-0a sym0-0b sym0-0c)
     :KEY-PL0-1 (sym0-1a sym0-1b sym0-1c))
    (:KEY-PL1-0 (sym1-0a sym1-0b sym1-0c)
     :KEY-PL1-1 (sym1-1a sym1-1b sym1-1c)))
  "This variable will have an ugly customize presentation.
Evaluating the following form:\n
 \(customize-variable '*tt--ugly-plist-custom*\)\n
presents a buffer named \"*Customize *Tt  Ugly Plist Custom*\".\n
Notice that for each plist the first pair of nested buttons [INS]
\[DEL] is indented deeper than subsequent pairs within the same plist
element. IOW, Instead of prsenting:\n
\[INS] [DEL] Plist:
             [INS] [DEL] Key: :KEY-PL0-0
                            Value: \(sym0-0a sym0-0b sym0-0c\)
             [INS] [DEL] Key: :KEY-PL0-1
                            Value: \(sym0-1a sym0-1b sym0-1c\)
             [INS]\n
\[INS] [DEL] Plist:
             [INS] [DEL] Key: :KEY-PL1-0
                            Value: \(sym1-0a sym1-0b sym1-0c\)
             [INS] [DEL] Key: :KEY-PL1-1
                            Value: \(sym1-1a sym1-1b sym1-1c\)
             [INS]\n\n
We instead get a customize presentation like this:\n
\[INS] [DEL] Plist:
                            [INS] [DEL] Key: :KEY-PL0-0
                            Value: \(sym0-0a sym0-0b sym0-0c\)
             [INS] [DEL] Key: :KEY-PL0-1
                            Value: \(sym0-1a sym0-1b sym0-1c\)
             [INS]\n
\[INS] [DEL] Plist:
                            [INS] [DEL] Key: :KEY-PL1-0
                            Value: \(sym1-0a sym1-0b sym1-0c\)
             [INS] [DEL] Key: :KEY-PL1-1
                            Value: \(sym1-1a sym1-1b sym1-1c\)
             [INS]\n\n
This indentation style \"plist\" custom types is confusing.\n"
  :type '(repeat plist :value-type (repeat symbol))
  :group 'emacs)

(customize-variable '*tt--ugly-plist-custom*)

emacs-version => 23.2.1

--
/s_P\