Revision 24084

From Olivier and me:

Split the TGraph classes to move the graphics code to separate classes.
These classes will be moved to the hist directory once the classes will be ok.
The TGraphs are painted via the TVirtualGraphPainter class and its concrete
implementation TGraphPainter.
At the difference of the TH1 classes for which a different painter is created
when drawing each histogram, one single TGraphPainter class is created.
The TGraphPainter class does not have internal state, all the information
from TGraph is passed to the drawing functions via parameters.