folderIcon: Update folder icon after dropping

After dropping an application into the folder icon, the
list of applications is updated but the folder icon itself
is not.

Introduce BaseIcon.update() and call it from FolderIcon
when redisplaying.

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/664

js/ui/appDisplay.js

@@ -1331,6 +1331,7 @@ var FolderIcon = class FolderIcon {
     _redisplay() {
         this._updateName();
         this.actor.visible = this.view.getAllItems().length > 0;
+        this.icon.update();
         this.emit('apps-changed');
     }