[Feature] Add keyboard shortcuts for next _slide_ / previous _slide_

The current shortcuts (left) and (right) only switch to the next or previous Step, which can be e.g. a text fading in.

### Description

When there are a lot of steps in slides, navigating the deck can be quite slow.

### Proposal

Add a pair of keyboard shortcuts for switching to the next or previous slide. I would suggest to pick one among:

- A1. Arrow Up (back) and Arrow Down (next)
- A2. PageUp (back) and PageDown (next)

### Links / References

- https://formidable.com/open-source/spectacle/docs/presenting-controls

### Further ideas

- B. While we are at it, we might want to add shortcuts to go to the first or last slide of the deck. I would suggest to use the keys "Home" and "End".
- C. Further, I find the navigation dots are quite small. I believe it would make navigation easier if all of the navigation bar was clickable, and not just each small dot: Each click would be mapped to the closest dot.