Add new mode context-menu-mode and use it in info.el and goto-addr.el

Add new mode context-menu-mode and use it in info.el and goto-addr.el

* lisp/mouse.el (context-menu-functions): New defcustom.
(context-menu-overriding-function): New function.
(context-menu-filter-function): New defcustom.
(context-menu-map): New function.
(context-menu-undo, context-menu-region): New menu functions.
(context-menu-mode): New mode.

* lisp/info.el (Info-context-menu): New function.
(Info-mode): Add Info-context-menu to context-menu-functions.

* lisp/net/goto-addr.el (goto-address-context-menu): New function.
(goto-address-at-click): New command.
(goto-address-mode): Add goto-address-context-menu to context-menu-functions.