Firefox 59 removed some SVG path seg APIs which breaks path editing.
This change updates pathseg.js to the latest version which contains a fix for Firefox 59. I checked that Firefox 58 (pre-api-change), Firefox 59 (post-api-change), Chrome 65, and Safari Tech Preview can all edit paths correctly with this change. This change has a lot of diffs due to minor changes in pathseg.js since the first time it was imported into svgedit.
This fixes issue #216