general: Rename "application" to "app"

Several places in Shell use the word "application". However,"app"
is a more common term nowadays, so update strings accordingly.

https://gitlab.gnome.org/Teams/Design/initiatives/-/issues/123
https://gitlab.gnome.org/GNOME/gnome-shell/-/issues/6202

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2614>

js/ui/appDisplay.js

@@ -1149,7 +1149,7 @@ var BaseAppView = GObject.registerClass({
         if (this._items.has(id))
             this._items.get(id).navigate_focus(null, St.DirectionType.TAB_FORWARD, false);
         else
-            log(`No such application ${id}`);
+            log(`No such app ${id}`);
     }
 
     selectApp(id) {