add extra keyboard controls as per W3C recommendations for combobox

[evanwills]

Add four extra keyboard controls for as per recommendations from the W3C https://www.w3.org/WAI/ARIA/apg/patterns/combobox/#keyboardinteraction

1. Control + Home (optional): moves focus to the first row.
2. Control + End (Optional): moves focus to the last row.
3. Alt + Down Arrow (Optional): If the popup is available but not displayed, displays the popup without moving focus.
4. Alt + Up Arrow (Optional): If the popup is displayed:
   • If the popup contains focus, returns focus to the combobox.
   • Closes the popup.