[Runtime] Add application main document manifest handler

With this manifest handler we can access the application main document related
info directly from MainDocumentInfo instead of parsing the manifest manually.
An unit test for the handler is also added.
[Runtime] Basic application onLaunched event support

In order to follow the main document lifecycle events, we need to launch the
application through "onLaunched" event.

Different from Chrome onLaunch event
(http://developer.chrome.com/apps/app_runtime.html#event-onLaunched), we will
only support xwalk.app.runtime.onLaunched(function callback) without launchData
object. The callback will be called without any parameter when the application
is launched explicitly (from home screen launcher or from command line).
[Runtime] Add application onLauched event browser test