PDF easy scroll/page swipe

[Originally suggested in the discussion linked in the parent issue]

Scrolling forward by tapping on the screen once.

- Maybe like this: tapping once to go forward half a page, tapping twice to go back half a page.

- Some apps also do this: The screen is divided in half, the upper part is the upper half of the first page, the lower part is the lower half of the fist page. When you tap, the upper part changes to the next page while the lower part stays the same. When you tap a second time, the lower part also changes to the second page. Like that, you can continue playing without having to adjust your eyes.

We could have other ideas pitched in to see what's most useful.