vtkPlotBar:
Added documentation to Offset accessor/mutator functions.
Converted Width accessor/mutators to VTK macro.

TestBarGraph:
Correct header comments to use correct file name.
Cleanup variable names and comments.

Change-Id: Ifa99f578da78955465c66d29a7e547fb5c88b61b

Conflicts:

	Charts/Testing/Cxx/TestBarGraph.cxx