allowing composite charts to toggle the legend

When you compose stacked charts, the legend toggle stops working on the stacks.

In this fiddle, click on "B" on the first composite chart.  See that it works fine on the copies to the right:  http://jsfiddle.net/jrideout/RWs67/

This PR looks among the child stacks to toggle them if the composed charts have hidableStacks set to true.