extend display of maxspeed nodes [patch]

[Klumbumbus]

Currently every node with maxspeed=* shows a maxspeed icon with a maxspeed of 60, which is far from optimal.
This patch changes it, so it shows the actual value.

• The value is shown if the value of maxspeed is
  • only a number
  • a number followed by a whitespace and followed by
    • km/h
    • mph
    • knots
• If maxspeed=none a new icon is shown
• If maxspeed:variable is set or maxspeed=signals, the sign turns into black backgrund with white text, which then looks like a common ​LED sign.

All other cases should be tagging errors, therefore no sign is displayed.

​Source of new icon (license:PD)

Screenshot: