add support for displaying duration of an event

Created by: andreh12

It would be nice to see the duration of an event in the 'Event details' popup of the browser.

As a first step (as far as I understood) could RaportAPI also add the value returned by Event.getDuration() to result (around here https://github.com/cmsdaq/DAQExpert/blob/dev/src/main/java/rcms/utilities/daqexpert/servlets/RaportAPI.java#L73).