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/wanda.js

@@ -71,8 +71,7 @@ const WandaIconBin = new Lang.Class({
     Name: 'WandaIconBin',
 
     _init: function(fish, label, params) {
-        this.actor = new St.Bin({ style_class: 'search-result-content',
-                                  reactive: true,
+        this.actor = new St.Bin({ reactive: true,
                                   track_hover: true });
         this.icon = new WandaIcon(fish, label, params);