searchDisplay: Add ListSearchResult and ListSearchResults

These are for all search results except apps (and Wanda).
We also simplify a bit the packing of search results, which removes some
ugly code in navigateFocus() where we needed to call
st_widget_navigate_focus() twice, since the grid icon was composed by
two nested boxes, both focusable.

https://bugzilla.gnome.org/show_bug.cgi?id=681797

js/ui/appDisplay.js

@@ -403,12 +403,6 @@ const SettingsSearchProvider = new Lang.Class({
         pref.activate();
     },
 
-    createResultActor: function (resultMeta, terms) {
-        let app = resultMeta['id'];
-        let icon = new AppWellIcon(app);
-        return icon.actor;
-    },
-
     launchSearch: function(terms) {
         // FIXME: this should be a remote search provider
         this.appInfo.launch([], global.create_app_launch_context());