Refactor model: overhaul starting and ending times of jobs (calculation and fitting)

This MR introduces the use of QDateTime for starting and ending times of jobs instead of QString. In the future the "duration" attribute will become obsolete because it can be easily calculated if the starting and ending times are available in a usable format. The times are saved in ISO format in the SessionItems and are currently displayed as such in the job view, but since that is overhauled too, this will be corrected soon.