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/search.js

@@ -154,9 +154,6 @@ const SearchProvider = new Lang.Class({
      * Search providers may optionally override this to render a
      * particular serch result in a custom fashion.  The default
      * implementation will show the icon next to the name.
-     *
-     * The actor should be an instance of St.Widget, with the style class
-     * 'search-result-content'.
      */
     createResultActor: function(resultMeta, terms) {
         return null;