Don't preventDefault on mousedown events

This prevents focus/blur events from getting dispatched,
which was the root cause of #1295.

Fixes #1295.

js/id/behavior/drag.js

@@ -59,7 +59,7 @@ iD.behavior.drag = function() {
             offset = [0, 0];
         }
 
-        if (touchId === null) d3_eventCancel();
+        if (touchId === null) d3.event.stopPropagation();
 
         function point() {
             var p = target.parentNode || surface;

js/lib/d3.v3.js

@@ -1504,8 +1504,7 @@ d3.behavior.zoom = function() {
         w = d3.select(d3_window).on("mousemove.zoom", mousemove).on("mouseup.zoom", mouseup),
         l = location(d3.mouse(target));
 
-    d3_window.focus();
-    d3_eventCancel();
+    d3.event.stopPropagation();
 
     function mousemove() {
       if (d3.event.which === 0) {