Add an option to control how percent complete updates as duration changes

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Copied from the forum 
thread:http://forum.ganttproject.biz/view/20140317090117_674wpzs2b4vs4rem6b9obi4
6a#0

Suppose you have a 15 day task in your project. You start to implement the task 
in schedule.
After 5 days work your task is in schedule and you have increased "done" 
percentage to 33% (=5 days work done) by dragging the black line on UI. This is 
very handy.
But then you think about the remaining work and your schedule and realize that 
10 days is not enough to complete the task. You need 3 more days.
You try to increase the task by dragging the end of it by amount of 3 days. 
This is again very handy except that the "done" percentage remains at 33% which 
means that you just increased the amount of work done by one day. Of course you 
can fix this and manually drag the "done" percentage to 28% which is appr 5 
days. This is however error prone and could be avoided by (simple?) 
functionality addition.

Original issue reported on code.google.com by dbarashev on 17 Mar 2014 at 10:08