Compatibility of track editor with non-circuit tracks

[alexphieler]

• When doing the lane assignment, the editor assumes, that the track is a circuit, i.e. that the last cone of a lane is connected with the first one. Currently the lane assignment is also finished only when this connection has been made for both lanes.
• The timekeeping line for the finish is not highlighted.

The kind of track should be should be selectable in the editor and the above points addressed.