Emit Component.onDestruction before context is invalidated

When a component no longer has any live references, emit the
destruction signal immediately so that handlers are run before the
associated V8 resources are invalidated.  Also, when the root
context of the engine is destroyed, emit the destruction signal
before destroying any resources needed to process the resulting
binding invocations.

Change-Id: I722dd6e4b60c499b533fc45e33b61e95bca6187f
Reviewed-by: Michael Brasser <michael.brasser@nokia.com>