altTab: use AltTab prefix on AppIcon class

There's an AppIcon class on appDisplay, so it won't be easy to discern
the one we're going to add in altTab.

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/700

js/ui/altTab.js

@@ -648,8 +648,9 @@ class WindowCyclerPopup extends CyclerPopup {
     }
 });
 
-var AppIcon = GObject.registerClass(
-class AppIcon extends St.BoxLayout {
+var AppIcon = GObject.registerClass({
+    GTypeName: 'AltTab_AppIcon'
+}, class AppIcon extends St.BoxLayout {
     _init(app) {
         super._init({ style_class: 'alt-tab-app',
                       vertical: true });