ENHANCED: highlight holes in incomplete terms

ENHANCED: highlight holes in incomplete terms

* sweeprolog.el (sweeprolog-predicate-completion-at-point): fontify
holes when inserting them.
(sweeprolog-analyze-start-font-lock): reset font-lock-face in analyzed
region.
(sweeprolog-analyze-fragment-to-faces): fix syntax error handling,
don't highlight holes as it is done by...
(sweeprolog-analyze-end-font-lock): new function, highlights holes in
analyzed region when sweeprolog-highlight-holes is non-nil.
(sweeprolog-analyze-region-end-hook): add it.
(sweeprolog--hole): fix text property used for setting stickiness.