ADDED: short description for tokens on mouse hover (help-echo)

ADDED: short description for tokens on mouse hover (help-echo)

* sweep.pl (sweep_predicate_dependencies/2): new predicate.
* sweeprolog.el (sweeprolog-enable-help-echo): new user-option, when
non-nil sweeprolog-analyze-region-start-hook and
sweeprolog-analyze-region-fragment-hook are extended with...
(sweeprolog-analyze-start-help-echo)
(sweeprolog-analyze-fragment-help-echo): new functions, manage the
help-echo text property for Prolog text.
* README.org ("Hover for Help"): new section about help-echo.