ENYO-3045: when the Phonegap build panel is displayed, the default signing key of each plateform is selected.

A modifications are also done to improve the usability of this component :
* The message "Loading signing keys ..." is displayed while the Ajax request to get the user data is pending
* The inputs "key password" & "storage key password" and the button "save" are shown only when a signing key is selected.
* If the Ares user is authentified with a Phonegap account "A" > change to the Phonegap account "B" > comeback to the account "A" . Then the existing signing keys are not recreated.
* If the Ares user is authentified with a Phonegap account "A" which contains a set of signing keys for a given platform & switch to the Phonegap account "B" which contain also a set of signing keys in the same platform. Then the picker contains only the signing keys of the account "B".