Datepicker using Globalize 1.x

:boom: WIP...

The goal is to have Datepicker working with Globalize 1.x.

Depends on
- [ ] https://github.com/jquery/jquery-ui/pull/1316 `fnagel:calendar-value-pr`;

Todo
- [x] Destroy translation data (weekHeader, prevText, nextText , datePickerRole, currentText). Create user options instead;
- [x] Move `external/date.js` into `ui/` (see #1260 (comment));
- [ ] Fix RTL LTR direction (available at `cldr.main("layout/orientation/characterOrder")`, values are `left-to-right` or `right-to-left`);
- [ ] Get currentText: "Today" from locale.
- [x] Fix Use narrow day names (e.g., "ﺥ" EEEEE) if short (e.g., "ﺎﻠﺨﻤﻴﺳ" EEE) if too lengthy;
What name we should use to export `_Date`? Should we place this object inside `ui/calendar.js`, so we don't have to expose this class publicly? Note it's not used anywhere else besides calendar.