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/shellDBus.js

@@ -160,7 +160,7 @@ var GnomeShell = class {
             invocation.return_error_literal(
                 Gio.DBusError,
                 Gio.DBusError.FILE_NOT_FOUND,
-                `No application with ID ${id}`);
+                `No app with ID ${id}`);
             return;
         }