[webapp-container] Add a command-line option to allow webapps to specify a custom UA string

It is already possible to override the user agent for a given webapp. The process is bit more involving than adding a command line argument though:

- you need a webapp-properties.json file local to your webapp click package OR a manifesty.json file a unity-webapp-* folder,
- such a json file must have the listed properties:

{
    "name": "<yourwebapp>",
    "homepage": "<homepageurl>",
    "domain": "<domain>",
    "includes": []
}

- you must add a "user-agent-override" property to the file above with the proper user agent,
- you must also add --webappModelSearchPath=. to your command line,

Since all this is a bit cumbersome, I added a branch here that simplifies the process.

This bug was fixed in the package webbrowser-app - 0.23+14.10.20141003-0ubuntu1

---------------
webbrowser-app (0.23+14.10.20141003-0ubuntu1) utopic; urgency=low

  [ CI bot ]
  * Resync trunk

  [ Alexandre Abreu ]
  * Add a simple mechanism (simpler than the one already in place) to
    override user agents for a given webapp. (LP: #1370975)

  [ Alberto Mardegan ]
  * Fix handling of host cookies

  [ Olivier Tilloy ]
  * Ship a PNG icon instead of an SVG, and install it under the app’s
    own data dir. (LP: #1365408)
  * Make an autopilot test more robust by ensuring that the drawer menu
    is dismissed but not clicked. (LP: #1365304)
  * Persistently cache favicons on disk to avoid re-downloading them
    across application runs. (LP: #1340585)
  * Ensure that at most 2 webviews are instantiated at all times, to
    reduce memory consumption. (LP: #1376418)
 -- Ubuntu daily release <email address hidden> Fri, 03 Oct 2014 14:42:27 +0000

Opening an Ubuntu RTM task to put the fix back on the radar for an OTA-1 update