ENYO-1712: Per-application key-signing choice

- Fetches keys configured within build.phonegap.com account & proposes
  a list of them in the ProjectProperties PhoneGap tab.
- PhoneGap project properties UI kind is now in a dedicated phonegap-
  specific file.
- TODO: same split should happen for ProjectConfig & its associated
  JSON schema.
- TODO: Offer key credentials text inputs & store the result to unlock
  the keys.

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