change how the flip operation detects non-numeric direction fields

[k-yle]

see openstreetmap/id-tagging-schema#2109 (review)

now we hardcode number instead of combo/radio, so make it easier to switch between those two.

[matkoniecz]

note that one direction PR was already merged, see openstreetmap/id-tagging-schema#2081

[k-yle]

yeah, so therefore we can test this by creating a vertex with traffic_sign=yes and pressing v which should automatically add direction