ENHANCED: 'sweeprolog-find-predicate/module' improvements

ENHANCED: 'sweeprolog-find-predicate/module' improvements

Allow finding modules and predicates in another window with prefix
argument.  Also improve prompt for 'sweeprolog-find-predicate'.

* sweeprolog.el (sweeprolog-read-predicate-prompt): Remove trailing
colon and space.
(sweeprolog-predicate-visible-p-function): New user option.
(sweeprolog-read-predicate): Use it, and format PROMPT with predicate
at point as the default argument.
(sweeprolog-find-module)
(sweeprolog-find-predicate): Add optional argument OTHER-WINDOW.
(sweeprolog-describe-predicate): Adapt PROMPT argument passed to
'sweeprolog-read-predicate'.

* README.org (Finding Prolog Code): Expand.