PopupWidget: Fix reverse tabbing order when exiting popup
PopupWidget: Fix reverse tabbing order when exiting popup

Keyboard navigation via tabbing can exit/close the popup
after tabbing past the last element. But it doesn't close
the popup when navigating in reverse order via shift-tab.
This fixes it.

Bug: T259912
Change-Id: I62f52320aa665ca5a50faff59bf60266135792b6

wikimedia/oojs-ui,wikimedia/oojs-ui,wikimedia/oojs-ui,wikimedia/oojs-ui