feat: add scroll view up/down, without moving selection

[sbillig]

This separates scrolling from item selection, so the view can be scrolled down without moving the selection down. This is useful when viewing large change hunks that don't fit within the terminal viewport.

The default keybindings are ctrl+y to scroll up, ctrl+e to scroll down. Mouse/trackpad scrolling works well with the config setting mouse_support = true.

Checklist

• The modified code has some test-coverage (where applicable).
• make test is passing (this is what CI runs).
• New unreleased features/fixes/styling and performance improvements are documented via git: feat: / fix: / style: / perf:. Or e.g. perf(highlighting):.
  See https://github.com/altsem/gitu/blob/master/docs/dev-tooling.md