Add datepicker based on dialog pattern for issue 1046

[jongund]

Adds date picker dialog example to the modal-dialog directory for issue #1046.

Preview Link

Preview link for datepicker dialog

---

Preview | Diff

[spectranaut]

Found a user interaction bug: If you click on the calendar icon, the dialog opens but focus remains on the calendar icon.
This is mostly odd because a user might click the date then try using left/right arrows, but you still have to type "ENTER" or "SPACE" or "DOWN ARROW" before the focus will move from the icon (even though the dialog is open). The date is also not in the tab sequence until you send on of those keys.

[jongund]

Typically clicking on a button moves focus to the button, not somewhere else. Since the user clicked on the button to open the dialog, mostly likely they will be using the mouse to select a date.

But it is something we should ask Matt about to see if he thinks it is something the group should discuss and review.

[mcking65]

@jongund, It's working much better after the last set of commits. I found a few more things to change.

[mcking65]

@jongund, I pushed a few changes to the introduction.

One other change that I didn't include in my review ... currently the button is named "Calendar". I think we should change it to "Choose Date".

[jongund]

@mcking65 @spectranaut @a11ydoer I have made the changes requested yesterday and in the pull request to the datepicker dialog example so it should be ready for review and adding additional regression tests

[mcking65]

@jnurthen, thank you for the review and edits!! We're ready to ship this one.