Please add elapsed print time to event placeholders

[Lenbok]

I would like to hook up a command to the PrintDone event that includes the elapsed time as part of the notification. Could you please add that to the set of available placeholders? (currently there is progress in percent, and current time, but no elapsed time).

[foosel]

I've added it to the event in the devel branch, where the event payloads have significantly changed (see here). Please understand that I won't add it to the master branch, since the method of handling payloads there would make it signficantly harder to access this data at the time of event processing, and since the devel branch will eventually be merged unto the master branch, that would be wasted time and effort.