Hi Markus, as you have noticed there is no stock slideshow widget in apostrophe-lean-frontend. This is because our own developers nearly always wind up making custom slideshow widgets and using other slideshow js plugins anyway. So it’s a bit of an open question for us whether a default slideshow widget for lean frontend (and for 3.x) even makes sense. Although perhaps as a matter of marketing it to new developers it would be necessary to ship something.
Contributions to apostrophe-lean-frontend are of course welcome. Take a look at how the video widget is reimplemented in that module, including the option to enable pushing it at all, of course. But you might find you want to just follow that pattern and write your own player function that utilizes the js slideshow plugin of your choice. There are so many great ones.