[folgutter addon] Respond to swapDoc event asynchronously

Since the viewport info isn't updated until the redraw, and the event
fires before the redraw.

See https://discuss.codemirror.net/t/whats-wrong-fold-gutters-do-not-show-up-after-swapdoc-call/580

addon/fold/foldgutter.js

@@ -20,7 +20,7 @@
       cm.off("viewportChange", onViewportChange);
       cm.off("fold", onFold);
       cm.off("unfold", onFold);
-      cm.off("swapDoc", updateInViewport);
+      cm.off("swapDoc", onChange);
     }
     if (val) {
       cm.state.foldGutter = new State(parseOptions(val));
@@ -30,7 +30,7 @@
       cm.on("viewportChange", onViewportChange);
       cm.on("fold", onFold);
       cm.on("unfold", onFold);
-      cm.on("swapDoc", updateInViewport);
+      cm.on("swapDoc", onChange);
     }
   });