New public CalendarDateRange (end<start throws), CalendarMode {Month,Year},
CalendarSelectSource, CalendarCellType, CalendarCellContext, and the three
event args live in the top-level AtomUI.Desktop.Controls namespace. Rename
DatePicker test aliases to Picker* to avoid collision with these new types.
Move DateTimeHelper, CalendarExtensions, HeadTextButton, CalendarDateRange,
CalendarMode, CalendarSelectionMode, and calendar event args out of the top-level
AtomUI.Desktop.Controls namespace so the new Calendar can own it. Add xmlns
mappings and update DatePicker test usings. DatePicker behavior unchanged.