ENYO-2788: [Do not merge] Check the existance of a package before downloading it from Build status UI

#### Goal
The purpose Of this pull request is to confirm the code design on the widget **Build Status**, especially on the part of checking the existence or not of the built package to be downloaded from this widget.

#### Current status on the widget Build Status
* The widget "Build status" is situated in the panel **Phonegap Build**, it contains two blocks:
- _**The platforms buttons:**_ each supported build platform is represented by an Icon button that change the color depending on the building status for the specific platform.
  - _**The status message container:**_  have a content depending on the status of the build.

* If the status of an application returned by Phonegap build service, the status message container show an Hypertext link **Download application** to perform a download action for the built package related to the platform

* By clicking on this Hypertext link, a verification will be done by the widget **Build status** to check if the package was already downloaded by the user : 
  - If the built package exist in the folder _<project_name>/target/phonegap_ then the message _Override the package <package-name> [yes] [no]_ . When clicking on the buttons **yes** or **no** nothing happens
  - If the build package does not exist in the previous folder, then the message _Downloading the package <package-name>_ will be displayed.


#### Note:
When this PR is treated, it will be closed & not merged.

Link to JIRA ticket : https://enyojs.atlassian.net/browse/ENYO-2788

Tested on Chrome Canary (Windows 7)
Enyo-DCO-1.1-Signed-off-by: Rafik Adiche mahmoud-rafik.adiche@hp.com