ADDED: right-click context menus in sweeprolog-mode

ADDED: right-click context menus in sweeprolog-mode

* sweeprolog.el (sweeprolog-context-menu-find-module)
(sweeprolog-context-menu-find-module-other-window)
(sweeprolog-context-menu-describe-module)
(sweeprolog-context-menu-find-file)
(sweeprolog-context-menu-find-file-other-window)
(sweeprolog-context-menu-describe-predicate): new commands, add to context menus by...
(sweeprolog-context-menu-for-predicate)
(sweeprolog-context-menu-for-module)
(sweeprolog-context-menu-for-file): new functions, used in...
(sweeprolog-context-menu-functions): new abnormal hook, used by...
(sweeprolog-context-menu-function): new function.
(sweeprolog-mode): add it to context-menu-functions.

* README.org (Context Menu): new section.