I have added:
- modified help text for opening trees and session
- a record (produced for a valid X/Y/Z + drawing option selection when
clicking the red button) will get now the same name as the histogram.
- the treeviewer constructors define a variable gTV that can be accessed
only from interpreter to get a pointer to the treeviewer. This can be
used to append one or more new trees by pointer : new method
AppendTree() implemented. This is explained in the help menu Open/Load


git-svn-id: http://root.cern.ch/svn/root/trunk@7648 27541ba8-7e3a-0410-8455-c3a389f83636