Add setters to BDate and BCalendarView

* BDate setters don't perform any validation, use with caution.
* BCalendarView setters do perform validation checks, and will adjust
the day so it fits the requested month or year.
* Add tests for the BCalendarView setters.

headers/os/support/DateTime.h

@@ -119,6 +119,10 @@ public:
 			int32				Month() const;
 			int32				Difference(const BDate& date) const;
 
+			void				SetDay(int32 day);
+			void				SetMonth(int32 month);
+			void				SetYear(int32 year);
+
 			int32				DayOfWeek() const;
 			int32				DayOfYear() const;