[feature] Implement overscroll: scroll past diff edges to switch to adjacent file

[Naios]

When the text diff is scrolled to the bottom and the user keeps scrolling down, the next file in the change list is selected. Scrolling up at the top selects the previous file and shows it from its end. Can be toggled with a new button next to "Syntax Highlighting" (enabled by default).

This allows to scroll through an entire compound diff quickly without clicking on each file.

The overscroll feature is a cheap-to-implement variant of the continous diff feature requested here:

• [UX][PR] 瀑布流diff #2312
• Feature Request: Continuous diff view showing all changed files in a single scrollable pane #2317