ENYO-3429: avoid circular reference in data transfered during an HermesFileTree DnD

Related JIRA: https://enyojs.atlassian.net/browse/ENYO-3429

Circular references were introduced in data object transfered during HermesFileTree DnD. Data object is also simplified to avoid this case.

Enyo-DCO-1.1-Signed-off-by: Vincent HERILIER vincent.herilier@hp.com