[webapp-container] UA Override from webapp-properies.json file is not set

[Impact]

 Webapp has the option to either specify a command line option to have a specific UA overriding the default or have a local webapp-properties.json file that defines the UA override such as:

{
    "name": "MyWebapp",
    "includes":["http://user-agent.me/*"],
    "domain":"user-agent.me",
    "homepage":"http://user-agent.me/",
    "user-agent-override": "Mozilla/5.0 custom-user-agent-string (unlike AnyOtheBrowser)"
}

This currently does work.

[Test Case]

- from qtcreator, create a webapp,
- update the Exec line from the desktop file to
      Exec=webapp-container --enable-back-forward --store-session-cookies --webappModelSearchPath=. %u
- create a webapp-properties.json file in the project folder with the following content:
{
    "name": "MyWebapp",
    "includes":["http://user-agent.me/*"],
    "domain":"user-agent.me",
    "homepage":"http://user-agent.me/",
    "user-agent-override": "Mozilla/5.0 custom-user-agent-string (unlike AnyOtheBrowser)"
}
- make sure that the webapp automatically browses to http://user-agent.me/
- make sure that the UA override is the same as the one listed above,

[Regression Potential]

Make sure that the window title is properly set to the webapp name.

Note:
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this is not a dup of https://bugs.launchpad.net/webbrowser-app/+bug/1379497 but a regression