This is a bit unfortunate architecturally--ideally, all such notifications of change in an extension's status should be handled through the model (which would dispatch a change event upward to the view), and the model should in turn get that information from ExtensionManager. The problem is that the installation flow really lives fully in InstallExtensionDialog, when it should really live in ExtensionManager instead; we just never got around to refactoring that out. We don't necessarily need to do that right now, but we'll have to do it once we go to asynchronous installation.

That said, perhaps we could clean this up a little by having the InstallExtensionDialog notify the model directly instead of the view when the installation is done. (`handleNewInstall()` just ends up calling the model anyway.)