ENYO-1712: PhoneGap.ProjectProperties - use static members appropriately

Enyo-DCO-1.1-Signed-off-by: Francois-Xavier KOWALSKI <francois-xavier.kowalski@hp.com>
ENYO-1712: working keys load/store across platforms

- Store keys ID's in project.json (phonegap section)
- Store key description & credentials in Browser localStorage
- Update key Credentials directly from the PhoneGap pane in the
  ProjectProperties window.

Enyo-DCO-1.1-Signed-off-by: Francois-Xavier KOWALSKI <francois-xavier.kowalski@hp.com>