[dojo.data] add JsonItemStore support for hierarchical data files

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Make it possible to have JSON data files with item hierarchies. For example:

{ items:[
	{ name:'Africa', countries:[
		{ name:'Egypt', capital:'Cairo' },
		{ name:'Kenya', capital:'Nairobi' },
		{ name:'Sudan', capital:'Khartoum' }]},
	{ name:'North America', countries:[
		{ name:'Canada', population:'33 million', cities:[
			{ name:'Toronto', population:'2.5 million' }]}]}
]};

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(In [9484]) fixes #3627 fixes #3628

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