Revert "Revert "workaround for some drags not being accepted""

This reverts commit e7046ba398.

Without this change, dragging images from Google searches doesn't work on the
latest Chrome (tested on macOS).

web/editor.js

@@ -167,6 +167,9 @@ 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;