Provide API methods for hot folder directories

Two new API methods are required that return the directory of the currently configured hot folder:

a) spooler.hot_folder_base() returns e.g. c:/scheduler/config/live for a standard Windows installation
b) spooler_job.hot_folder() returns the current hot folder of a job, e.g. c:/scheduler/config/live/samples if the current job is located in the "samples" hot folder. For jobs that are not located in hot folders this method returns an empty string (not a null value).