Solving the "Disallow public access to addons listed in the apps menu" h...

Fixes friendica/friendica#741

Add plugin_is_app function in include/plugin.php to check if a plugin is an app or not (check the existence of an 'app_menu' hook)

Populate the app menu conditionally ( is the user logged or not, are apps private ) and disallow apps execution if apps are private and the user is not logged.