Add [Clear] button to DatePicker based components.

[berecik]

• Added nullable property to Calendar, DatePicker, DateRangePicker, and WeekPicker that allow set value to null.
• Property nullable is as a default set to false.
• If the nullable property is true, the Calendar shows the [Clear] button to be able set the value to null.
• If the value is null, the onChange function is call with null as an argument.