Fix quadratic behaviour in when selecting an insert completion
Fix quadratic behaviour in when selecting an insert completion

Use the fast, linear time SelectionList::insert algorithm instead of
an ad-hoc one that needs to call SelectionList::update after each
modification of the buffer.

Fixes #1417

occivink/kakoune,lenormf/kakoune,casimir/kakoune,lenormf/kakoune,alexherbo2/kakoune,Somasis/kakoune,occivink/kakoune,Somasis/kakoune,lenormf/kakoune,mawww/kakoune,jkonecny12/kakoune,jjthrash/kakoune,jjthrash/kakoune,jkonecny12/kakoune,danr/kakoune,mawww/kakoune,alexherbo2/kakoune,casimir/kakoune,mawww/kakoune,Somasis/kakoune,danr/kakoune,lenormf/kakoune,occivink/kakoune,occivink/kakoune,danr/kakoune,mawww/kakoune,casimir/kakoune,Somasis/kakoune,jkonecny12/kakoune,alexherbo2/kakoune,alexherbo2/kakoune,jjthrash/kakoune,jkonecny12/kakoune,casimir/kakoune,danr/kakoune,jjthrash/kakoune