workaround for some drags not being accepted

https://anki.tenderapp.com/discussions/ankidesktop/26688-drag-and-drop-not-always-working

web/editor.js

@@ -162,6 +162,9 @@ function focusPrevious() {
 }
 
 function onDragOver(elem) {
+    var e = window.event;
+    e.dataTransfer.dropEffect = "copy";
+    e.preventDefault();
     // if we focus the target element immediately, the drag&drop turns into a
     // copy, so note it down for later instead
     dropTarget = elem;