Only extract goals to separate predicates, not data terms

Only extract goals to separate predicates, not data terms

* sweeprolog.el (sweeprolog-context-menu-for-region)
(sweeprolog-maybe-extract-region-to-predicate): Check that the region
starts in a callable position.
(sweeprolog-extract-region-to-predicate): Reanalyze the buffer as a
last step.
* sweep.texi: Adjust.