Allow showing show-paren context in an overlay

Allow showing show-paren context in an overlay

* lisp/paren.el (show-paren-context-when-offscreen): Add new
possibility `overlay'.
(show-paren--context-overlay): New defvar.
(show-paren--delete-context-overlay): New function.
(show-paren--show-context-in-overlay): New function.
(show-paren-function): Handle the new `overlay' case.
* lisp/emacs-lisp/eldoc.el (eldoc-display-message-no-interference-p):
There's no interference if `show-paren-context-when-offscreen' is
overlay or child-frame.