Id1342

importtask bugs:

1) importtask changes task duration, because it sets end=start + duration.
2) datediff wrong because diff is taken between old project start date and new (!!) project start
3) if new start date is shifted to next working day, that shortens the task because end date is not shifted the same number of days

fix: 

1, 2) datediff is new project start date - old project start date  #1
enddate (new) to start day difference is still kept
3) if start date is shifted due to weekend, enddate is also.

consequence: in mew project, number of working days between start and end is same as in old project.