Better chart positioning during zoom

[dbarashev]

In case when we zoom in using a mouse wheel, or if a pointer is located somewhere in the chart area, it makes sense to position the chart so that the tasks that are under the pointer remain under the pointer and do not scroll away. This will make zoom more predictable.

Discussion on the support portal:
https://help.ganttproject.biz/t/improvement-scroll-wheel-zoom/11250