Provide slideshow mode actions with information of the state of zoom/pan.

I've added this to make it easy to have an action that crops an image to what's currently visible in the window - this way it's easy to crop by zooming/panning to desired image portion and then executing the action.

P.S. Commit e220395 required a bit of unfortunate copy/pasting, not sure if it's worth trying to create a shared helper function instead.