Resolve #2568785 "Adding an administrative description for menu items"

This is a proposed solution to handle administrative description for menu items and remove some hard-coded lines in MenuForm.php. The idea is to add a new field "admin_description" for menu items so description like "Logged in users only" or "Log in for anonymous users" presently hard-coded can be managed in a unified pattern. This introduces the possibility to add Administrative description for other menu items coming from the Menu UI or from Yaml/coded plugins.