ENYO-2513: Top file selection in project's properties for the preview settings

Related JIRA: https://enyojs.atlassian.net/browse/ENYO-2513

Top file selection in project's properties for the preview settings
The file chooser cannot be set currently on the related project filetree, so the service has to be selected, and then, the project folder has to be retrieved.
Selecting the file, set the top file entry in the project preview tab.

Checked on Windows 7 : Chrome

DO NOT MERGE

Enyo-DCO-1.1-Signed-off-by: Vincent HERILIER vincent.herilier@hp.com