Delete scene if you aren't going to use it.

Delete scene if you aren't going to use it.

Also make QDeclarativeOpenMetaObjectType zero its engine when the engine
dies, because this was leading to problems when the scene wasn't
deleted. Also fixes a few other potential deletion problems.

Change-Id: I37fa2d1a0b438a6ea7d8cf4c5c9d0bd869faa959
Reviewed-on: http://codereview.qt.nokia.com/2250
Reviewed-by: Alan Alpert <alan.alpert@nokia.com>