In the widget, there is a field for "Slideshow". It took me a while to figure out that this was the slug. Maybe you could change the field name to "Slideshow Slug" to be more clear? But even better, why not make this a dropdown list of all the available slideshows? The first choice in the dropdown list could be "All" (the default if one leaves the field blank).
It just seems like that would be one more thing to do which would make your already easy to use slideshow even easier.