presage: Better handle presage's predictions

Presage knows the difference between the prediction and what should actually be completed. So if the user e.g. types "time" then the predictions are "time" and "timetable". The actual completions are " " and "table ". Make use of that feature so e.g. deleting backwards into a word works better.

This is based on !238 (merged) and thus a bit hard to review atm (hence marking as draft until the base commits land).