bug#20664: 25.0.50; Customize: Link option or face name to the source definition

[Drew Adams]

> >> With this patch, "RET" works as expected, while "<mouse-1>" is
> >> giving an error for a reason I don't understand.
> > Bug#20398?
> 
> No, this is likely caused by the fact that `Custom-mode' binds
> "<down-mouse-1>" to the incompatible (with buttons) `widget-button-click'.
> And even with 'follow-link t, "<down-mouse-1>" supersedes "<mouse-1>",
> which is actually bound to `push-button'.
> 
> I think if `Custom-mode' didn't bind "<down-mouse-1>", it would
> translate to "<mouse-1>" and properly call `push-button'.

FWIW: With `emacs -Q', setting `mouse-1-click-follows-link' = nil,
`mouse-2' gives the same error message, plus an additional error saying
that `symbol' is an bound variable.

widget-button-click: Wrong type argument: consp, #<overlay from 259 to 272 in 
*Customize Face: Dired Header*>
Symbol's value as variable is void: symbol