DrillDown stack is not cleared when one nagivates away from a page with a chart and then come back later, causing weird display behaviors & AIOOB Exception

[echarlus]

When a chart has drill down enabled and is currently in a drill-down state, the drilldownStack in ChartServerRpcImplementation contains the Series that have been drilled-down to.
If the chart is embedded in a TabSheet and one navigates away to another sheet while the chart had been drilled-down and then one come back to the tab containing the chart, the chart will be displayed in it's original (non drilled down) state instead of in the previous drill-down state. However the stack will still contain the previous drilled-down series which will cause the crash/problem when ChartServerRpcImplementation.resolveSeriesFor(index) is called from onChartDrilldown() because it will use the series on the stack rather than call getConfiguration().getSeries().get(seriesIndex);
Thus the wrong series is returned and it causes issues.
One way to fix this is to add a new method to chart to allow to programmatically reset the drill down state when the view is displayed.

[alvarezguille]

Fixed by #502