[BUG #8079] First version of the date picker widget.
[BUG #8079] Added a custom 'render' method to be able to call the render method on the datepicker and to get the calendar rendered. Additionally set the connected input elements to 'readonly'
[BUG #8079] Updated implementation to make sure the 'getCalendar' method works also for a collection of datepickers.
[BUG #8086] Extended the calendar widget with CSS classes to be able to style the container and controls.

This includes CSS classes for:
* table container
* cells for prev and next button
* weekday and weekend
* cell for day names
* cell for month name
[BUG #8079] Added API documentation sample.
[BUG #8079] Added 'dispose' method and minor changes to the event listeners.