Add access capability to Moodle Extras menu in 2.0 core

request expansion of Moodle Custom Menu (MDL-22398) to include a visibility/access capability on the system level.

Taken from thread <http://moodle.org/mod/forum/discuss.php?d=149803>

) check if a user has the capability to view the menu, something like this:

if ($hascustommenu && has_capability(moodle/site:viewcustommenu)) {
//display custom menu
}

so that I don't have to use the "legacy function" to hide it from guests/roles:

if ($hascustommenu && !(has_capability(moodle/legacy:guest || has_capability(moodle/legacy:student))) {
//display custom menu
}