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. The logic now works as follows:

Whenever a document is closed, we call `_updateOpenDocuments`. Since documents can get closed "in the background" without the active document changing, this function does not change the menu state most of the time. (If a close necessitates a menu change, we will ALSO get an `imageChanged` activation event, which will update the menu.) However, if this function discovers there are now zero open documents, it sets `_currentDocumentId` to `null` and handles calling `_setMenuState` to disable the menu. 

Whenever a document gets activated (which could either be switching documents or opening a new document), we call `_handleCurrentDocumentChanged` (which perhaps could be renamed to `_handleDocumentActivation`). This method in turn calls `_updateOpenDocuments`, and then uses the results from that call to set the menu state appropriately.

Also, this PR (hopefully) clarifies some of the logic around `_updateOpenDocuments`.

And finally, it removes dead code for the nonexistent `documentClosed` event.