Avoid excessive analysis in presence of unbalanced quotes

Avoid excessive analysis in presence of unbalanced quotes

* sweeprolog.el (sweeprolog-analyze-fragment-fullstop): new function,
adds a dedicated text property to fullstops.
(sweeprolog-analyze-region-fragment-hook): add it.
(sweeprolog-at-fullstop-p)
(sweeprolog-end-of-last-fullstop, sweeprolog-end-of-next-fullstop):
new functions.
(sweeprolog-end-of-top-term): use them for restricting to region
between last fullstop and next fullstop.