[frontend performance] link rendering setting

Offer a setting to disable link rendering at all, or only render links on hover.
This makes the nodenet editor less painfully slow when working with larger nodenets
prevent ambigous hoverNode setting

if the hoverNode changed, unset hoverNode for redrawing
Update README.md
Merge branch 'master' of github.com:joschabach/micropsi2 into minecraft