ENYO-2771: avoid to create a file with a name that is already used in the same folder

Related JIRA: https://enyojs.atlassian.net/browse/ENYO-2771

Now it's impossible to create a file with a name that is already used in the same folder. 
In hermes filetree, favour is given to work with hermes node than file node.

Checked on Windows 7 : Chrome

Enyo-DCO-1.1-Signed-off-by: Vincent HERILIER vincent.herilier@hp.com