boolean values. (Modified quite a lot by Ashesh, before committing).
This patch also allow to set the header class in the backgrid through
setting 'cellHeaderClass' property of the schema object for any of the
node. It will allow us to set the different behaviour of the cell in the
list. One of the test case for it is: this will allow us to set the
different width of each column in the list using CSS class(es).
