When a junction changes, try and preserve custom Lane Arrows

[kvakvs]

Subtask of EPIC #43

Description

When a junction has some custom lane arrows set, and the player upgrades or changes one of the road segments in another way (bulldoze etc), try and preserve as much of user's work as possible by throwing away impossible turns, resetting the junction lane arrows, and then applying the remaining custom arrows again. This will eliminate wrong turns and preserve still possible turns set up by the user.

Impossible turn is a circle slice where there are 0 turns available in that direction.

Existing Behaviour

Bulldozing or upgrading a road segment on a junction resets the lane arrows.

Desired Behaviour

Bulldozing or upgrading a road segment on a junction only resets lane arrows leading to that segment, while preserving the other directions.

[originalfoo]

See also: #43

The display aspect is particularly important so users know their action is going to have road/rail config consequences.

[originalfoo]

Also test "not quite a road" structures such as toll booths and on-road monorail stations - see #471