[Runtime] Implement onSupsend event support.

"onSuspend" event is sent to the main page just before it is closed.
This gives the application an opportunity to do some clean up.

When application process manager finds the last open window is closed,
it sends 'onSuspend' event to main page.
The application process manager is also an observer of 'finish onSuspend'
event so that it will close the main page after the 'onSuspend' event is
executed.