Add possibility to delay page loading after a step was finished

Closing a step after some user chose to finish the task is done asynchronously. Unless in case of time-taking automatic tasks, this usually only takes fractions of a second. However, the result page is usually rendered just in the moment before the step is closed, still showing the task and thus giving the user the feeling of a caching problem: It requires to reload the page to gain access the next step which one would expect to be available right away. This commit adds the possibility to set a property to delay the loading of the next page for some milliseconds to increase the probability ot get access to the next step just in turn.