DropdownWidget: Open menu on up and down arrow keys
DropdownWidget: Open menu on up and down arrow keys

Bug: T124825
Change-Id: I2e7b78a0eba612470176f7f7c78289fe057205a8
wikimedia/oojs-ui,wikimedia/oojs-ui,wikimedia/oojs-ui,wikimedia/oojs-ui