Optimized highlighting

Old code was really inefficient if you have many `textNodes` (what is the case in pdf.js text layer). Code forced DOM change for every node separately. It should probably be using a DocumentFragment to cache things somehow and then replace at once all. jQuery is internally doing that so using jQuery on all set improves things. On my tests I got a 40% improvement (instead of 1.9 s for whole page selection, 1.1 s).