Fix several small bugs in the notebook trust framework

1. The trust dialog is not called with the notebook context, so we have to use the closure to bind to a notebook event.

2. Replace != with !==

3. The trust_changed.Notebook event default handler expects the trust value to be value key inside an object.  In fact, the event handler is redundant with the code--both seem to set the trust value of the current notebook.

Needless to say, you should scrutinize these changes carefully :).  In particular, I'm not sure why you had != instead of !==.