Return drilldown data source info in request to /Widgets

Additional data to return.

Example:
```javascript
{
"children":[
	{
	"type":"pivot",
	"title":"",
	"key":737374165,
	"mdx":"SELECT NON EMPTY [Product].[P1].[Product Category].Members ON 0,NON EMPTY HEAD(NONEMPTYCROSSJOIN([DateOfSale].[Actual].[YearSold].Members,[DateOfSale].[Actual].[MonthSold].Members),2000) ON 1 FROM [HoleFoods]",
	"dataSource":"test.pivot",
	"drillDownDataSource":""
	},
	{
	"type":"DeepSee.LightPivotTable",
	"title":"",
	"key":629008112,
	"mdx":"SELECT NON EMPTY [DateOfSale].[Actual].[YearSold].Members ON 1 FROM [HoleFoods]",
	"dataSource":"LightPivot\/test.pivot",
	"drillDownDataSource":"Drill Options.dashboard"
	},
]
}
```