Conditional disabling of calendar days after instantiation

We needed a way to disable the days in the calendar after the DatePicker is instantiated. To support disabling the days in general, we added a callback that takes as parameter the Calendar representing the day and returns a boolean that is set to true if the day should be disabled and false otherwise. The callback is executed in the CalendarPickerControlSkin.java within the refreshDayButtonsVisibilityAndLabel() method, which is responsible for decorating the days. To support disabling the days 'after' instantiation, we made the aforementioned method public so that it can directly be called without instantiating a new DatePicker.