Enable arrow up/down keys to navigate sidebar

[raphaelschaad]

Seems useful for navigation, and feels nice on hold-keydown to quickly navigate the list up and down.

Should still make sense even if the app becomes a menubar app in the popover style.

[romannurik]

Thanks! One minor bug, when the search field is focused, pressing Up or Down shouldn't switch colors. You may be able to do something along these lines:

  if (document.activeElement.matches('input')) {
    return true;
  }

[romannurik]

Dup of #25