Solving the "Disallow public access to addons listed in the apps menu" has no effect bug

Adding plugin_is_app function in include/plugin.php for checking if a plugin is an app or not (checking the existence of an 'app_menu' hook)

Populating the app menu conditionaly ( is the user logged or not, are apps private ) and dissalowing apps running if apps are private and the user not logged
Merge pull request #747 from zered/solve_741

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