Add DatePicker control

DatePicker control to allow choosing and presenting a date.

The goal for JavaFX 8 is to have a simple DatePicker based on the new Date & Time API (JSR-310) in Java 8. This will both fulfill the need for a DatePicker, as well as help migrate people into properly using the new JSR-310 Date & Time API.

Requestors: the JSR-310 team, as well as the Java FX Controls team.

Size & Risk: The scope is fairly small, and we consider this low risk since there are no down-stream items that depend on this.

- Approximately 6 new classes: a Control class and a very small DateCell class, each with associated skins and behaviors.
- The diff is in the order of ~1000 lines.
- There may be a few (less than 10) new strings for l10n; these are mostly for date formatting where JSR-310 doesn't suffice.

Mitigation plan: if for any reason we decide this is not ready, we can easily pull it up until fairly late in the game.

Timeline: M8, mostly due to dependencies on the still-changing java.time API.