Switch to select2 library

Problem/Motivation

Sachini did a great job at implementing the autocomplete UI using Drupal core's jQuery UI, based on the combobox example from http://jqueryui.com/autocomplete/#combobox.

I looked if we could upgrade combobox to support categories as well, so that we can have two group of properties: the ones from the types and all the other ones. There is http://jqueryui.com/autocomplete/#categories but we would lose the advantage of the combobox.

Proposed resolution

In talking with Kevin O'Leary, https://ivaynberg.github.io/select2/ is a much better library to achieve this kind of UI. I think we should upgrade to select2, it seems to support what we're looking for pretty much out of the box. There is an issue discussing this for Drupal 8 core as well: #1271622: Seek a better autocomplete replacement for core (jQuery TokenInput / Select2 / Typeahead.js by Twitter).

Remaining tasks

Write a patch. To include an external library in Drupal 8, see how these modules do it:
- https://www.drupal.org/project/fullcalendar
- https://www.drupal.org/project/shadowbox

User interface changes

The new autocomplete widget would support multiple values, categories, and drop down.

API changes