Add in the ability to specify margins in theme, fixes #18079

This is a simple addition to the dojox/charting/Chart.js file to allow themes to specify chart margins.  It keeps backwards compatibility to the way it behaved previously.
yes, but this increases the danger to change the default in one of the instance only and create a discrepancy. If that is just for documentation purpose I guess it would be better in the example section of the SimpleTheme api doc?