SearchDisplay: handle certain result IDs specially

Allow the prefix 'special:' applied to result IDs to mark results
that should be always shown, even when they would overflow the
maximum results cap. This will be used by epiphany for the special
"Search the Web" result.

https://bugzilla.gnome.org/show_bug.cgi?id=707055

js/ui/appDisplay.js

@@ -877,6 +877,10 @@ const AppSearchProvider = new Lang.Class({
         callback(metas);
     },
 
+    filterResults: function(results, maxNumber) {
+        return results.slice(0, maxNumber);
+    },
+
     getInitialResultSet: function(terms) {
         this.searchSystem.setResults(this, this._appSys.initial_search(terms));
     },