Adopt tig's navigation key bindings

[leg100]

Currently in the split pane view, all the navigation keys only take affect on the currently active pane.

Consider adopting tig's approach:

• up/down arrow keys always move the currently selected item in the list pane.
• other navigation keys, such as pgup/pgdown, take affect on the currently active pane.

Taking such an approach allows the user to navigate both panes without switching the currently active pane back and forth.