Add new context-menu options for menus "File At Point" and "Version Control".

Add new context-menu options for menus "File At Point" and "Version Control".

* lisp/mouse.el (context-menu-functions): Add more options.
(context-menu-global, context-menu-local): Fix separators.
(context-menu-minor): Rewrite to support list of submenus.
(context-menu-vc, context-menu-ffap): New functions.
(context-menu-undo, context-menu-region): Fix separators.

* lisp/dired.el (dired-context-menu):
* lisp/info.el (Info-context-menu):
* lisp/net/goto-addr.el (goto-address-context-menu):
* lisp/net/eww.el (eww-context-menu):
* lisp/progmodes/prog-mode.el (prog-context-menu):  Fix separators.