ENYO-2512: PhoneGap settings for icon/splashscreen resources can be chosen with a fileChooser

Related JIRA: https://enyojs.atlassian.net/browse/ENYO-2512

* Icon/splashscreen resources for PhoneGap settings are now chosen with a fileChooser popup and related path is indicated as valid or not (like for top application file for the preview: https://enyojs.atlassian.net/browse/ENYO-2513).
* Settings management for PhoneGap properties has been changed to fit the settings management strategy used for ares/preview settings (setting/getting/discarding the project properties):
 * properties are saved in "project.json" as the "ok" button is clicked.
 * "cancel" button discards modified properties.
 * properties are reloaded from "project.json" each time the "Project Properties" popup is called.

Checked on Windows 7 : Chrome

Enyo-DCO-1.1-Signed-off-by: Vincent HERILIER vincent.herilier@hp.com