API functions for schedules

If an order uses a schedule, Order.run_time.xml returns <run_time schedule="schedule_name"/>.
Additionally the following functions are provided:

Order.run_time.schedule returns Schedule object (new) or null if no Schedule is assigned to the order

Schedule.xml returns and sets a schedule just like Run_time.xml

spooler.schedule(name) returns the Schedule objekt with the given name or null if it doesn't exist.