As we don't get notified by GConf about whether or not shotwell is the preferred image viewer (or whether or not it is installed), the check for suitable shortcuts in the dash had to be done every time the dash is opened. I had to create a new UBusMessage to achieve that.
The shortcuts on the HomeView first check if the default application is available. If not (e.g. because the preferred program was just uninstalled), some alternatives are tested. And if no suitable app was found, the shortcut is simply not displayed.