[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