Fix dbl-click handling on webkit-based browsers

d3.event.buttons is not as widely supported as I thought. Can
change this one instance as it is inside a click handler so
d3.event.button will be defined instead

editor/js/ui/view.js

@@ -1766,7 +1766,7 @@ RED.view = (function() {
         clickTime = now;
 
         dblClickPrimed = (lastClickNode == mousedown_node &&
-            d3.event.buttons === 1 &&
+            d3.event.button === 0 &&
             !d3.event.shiftKey && !d3.event.metaKey && !d3.event.altKey && !d3.event.ctrlKey);
         lastClickNode = mousedown_node;