Make the variables be held in a proper class.

In preparation for enumerations, a more complete handling of variable information
is required.  Thus create a VariableCollection and use it appropriately.  As
much as possible, the VariableCollection is pushed down the stack as much as
possible.

git-svn-id: https://groke.mcmaster.ca/svn/grad/colin/branches/TableTool_javization@11083 57e6efec-57d4-0310-aeb1-a6c144bb1a8b