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/overviewControls.js

@@ -405,7 +405,7 @@ class ControlsManager extends St.Widget {
 
         Main.ctrlAltTabManager.addGroup(
             this.appDisplay,
-            _('Applications'),
+            _('Apps'),
             'view-app-grid-symbolic', {
                 proxy: this,
                 focusCallback: () => {