Display show-paren-context-when-offscreen in child frame

Display show-paren-context-when-offscreen in child frame

* etc/NEWS: Extend section about the new
`show-paren-context-when-offscreen' custom option.
* lisp/paren.el (show-paren-context-when-offscreen): Allow special
value `child-frame'.
(show-paren-function): Handle `child-frame' value of
`show-paren-context-when-offscreen'.
(show-paren--context-child-frame): New defvar.
(show-paren--context-child-frame-redirect-focus): New function.
(show-paren--context-child-frame-buffer): New function.
(show-paren--context-child-frame-parameters): New defvar.
(show-paren--delete-context-child-frame): New function.
(show-paren--show-context-in-child-frame): New function.