[dnd] revert the change to only allow dropping on reactive actors

Workspace.actor is non-reactive, and there's probably some good reason
for this. So just have the add workspace button check its own reactivity.

js/ui/overview.js

@@ -511,6 +511,6 @@ AddWorkspaceButton.prototype = {
 
     // Draggable target interface
     acceptDrop: function(source, actor, x, y, time) {
-        return this._acceptDropCallback(source, actor, x, y, time);
+        return this.reactive && this._acceptDropCallback(source, actor, x, y, time);
     }
 };