appDisplay: Add translator comment for "All" filter

The correct translation for "All" depends on context in some
languages, so add a translator comment.

https://bugzilla.gnome.org/show_bug.cgi?id=637559

js/ui/appDisplay.js

@@ -151,6 +151,7 @@ ViewByCategories.prototype = {
         this._apps = apps;
         this._view.refresh(apps);
 
+        /* Translators: Filter to display all applications */
         this._addFilter(_("All"), -1);
 
         if (!sections)