Allow adding new values to a directory from select2 widgets

When choosing a value in a select2 directory widgets, it would be useful to allow users to add new values directly from here.

Description of the needed feature:

• when the user can add entries to this directory (access controlled by an action filter), show a button "add entry" next to the select2 input
• clicking on this button opens a fancybox showing the creation layout for entries of this directory
• when form in fancybox is submitted, the new value can be selected in the select2 input