Make file preview scrollable

[TD-Sky]

This Pull Request fixes/closes #2205.

It changes the following:

• New config item seek_up,seek_down in key_bindings.ron.
• Enable scrolling file preview without entering file.

I followed the checklist:

• [ ] I added unittests
• [ ] I ran make check without errors
• [ ] I tested the overall application
• [ ] I added an appropriate item to the changelog

[TD-Sky]

I have made the file previews scrollable in Files [3]. My purpose is gitui can scroll file without entering in all tabs.

[TD-Sky]

The recommended config for vim key bindings:

    seek_up: Some(( code: Char('k'), modifiers: "ALT")),
    seek_down: Some(( code: Char('j'), modifiers: "ALT")),