ENYO-2495: Ease DnD action by considering files like their folder parent node if it is valid

Related JIRA: https://enyojs.atlassian.net/browse/ENYO-2495

For DnD, files nodes are not valid targets, but their parent folder node could be a valid one. If it's the case, dragging a node over files of a valid folder will highlight the folder and dropping the node will be performed on the folder.

Checked on Windows 7 : Chrome/Chrome Canary/FireFox 21/Opera/Safari/IE10

Ready to be reviewed

Enyo-DCO-1.1-Signed-off-by: Vincent HERILIER vincent.herilier@hp.com