Improved list navigation

<kbd>up</kbd> or <kbd>shift+tab</kbd>: select previous item
<kbd>down</kbd> or <kbd>tab</kbd>: select next item
<kbd>enter</kbd>: insert item below selection
<kbd>shift+enter</kbd>: insert item above selection

Addresses (2), (4), (5), (6), and (7) of #208.
Addresses (2) and (3) of #20.