search: add API to get a list of remote providers

This will be used to reload them in case the configuration changes.

https://bugzilla.gnome.org/show_bug.cgi?id=687491

js/ui/remoteSearch.js

@@ -114,7 +114,7 @@ const RemoteSearchProvider = new Lang.Class({
         this._proxy = new SearchProviderProxy(Gio.DBus.session,
             dbusName, dbusPath, Lang.bind(this, this._onProxyConstructed));
 
-        this.parent(appInfo.get_name().toUpperCase(), appInfo);
+        this.parent(appInfo.get_name().toUpperCase(), appInfo, true);
         this._cancellable = new Gio.Cancellable();
     },