Change: Show 6 or 2 orientation buttons in NewGRF road stop picker as appropriate.

This replaces the normal method of masking unusable buttons.

Codechange: use better location for the "invalid" action sentinel value

It used to be a random sentinel for end-of-(widget-)list that was used to tell
that no action has taken place yet. Since the last action is practically the
widget that was pressed, add the sentinel to that enumeration.