altTab: Use correct actor in label height computation

Commit f2bd39b20 removed an intermediate bin, and now we use the
thumbnail bin instead of the label actor to compute the label
height, whoops.

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/804

js/ui/altTab.js

@@ -899,7 +899,7 @@ class ThumbnailList extends SwitcherPopup.SwitcherList {
                     // St.Label doesn't support text-align
                     x_align: Clutter.ActorAlign.CENTER,
                 });
-                this._labels.push(bin);
+                this._labels.push(name);
                 box.add_actor(name);
 
                 this.addItem(box, name);