Disable "Image Assets" menu if no documents are open.

To accomplish this, switch from using currentDocumentChanged events
to imageChanged events to see when the set of open documents changed.

Also, clarify (hopefully) some of the logic around updateOpenDocuments.
Merge pull request #208 from adobe-photoshop/jrb/iwehrman-v2-with-menu-disablement

Disable "Image Assets" menu if no documents are open