Class: MoodleQuickForm_searchableselector  - X-Ref

searchable select type element

Display a select input with a search textfield input on the top
The search textfield is created by the javascript file searchselector.js
(so when javascript is not activated into the browser, the search field is not displayed)
If ever the select can be reset/unselect/blank/nooption, you will have to add an option "noselected"
and manage this special case when you get/set the form data (i.e. $mform->get_data()/$this->set_data($yourobject)).