Bug 1076700 - [Smart Screen][Home] Basic key navigation feature

Oops. I found that `KeyEvent.key` is not following W3C standard for Gecko yet. the key is still "Up"/"Down"/... used from Gecko 23, with a warning in console `KeyboardEvent.key value "Down" is obsolete and will be renamed to "ArrowDown".` 

But KeyCode is also a deprecated standard so that we need to change it someday after. I would suggest let's just leave some comment, and jump to `KeyEvent.key` after Gecko synced with W3C standard.