Allow for showing custom columns around task bars

[GoogleCodeExporter]

One may choose to show default columns around task bars on Gantt chart with 
chart options. However, custom columns can't be shown this way.

Initially requested here: 
http://forum.ganttproject.biz/view/20130109124427_54nm4r4o9oyzcc5c3b08jd0k1#0

Original issue reported on code.google.com by dbarashev on 9 Jan 2013 at 1:09