Using a KAbstractFileItemActionPlugin allows to have more control over when actions are shown in context menus.
Right click any number of folders with no images in it -> Start a slideshow is not visible
Right click on any number of folder(s) containing at least one image -> Start a slideshow is visible
Same for files.
When the selection contains a mix of folders and files the action is not displayed.
Remaining question: how to remove the old SlideShow Service Menus ?