Add voxel snapping to rail planner

Don't change material for unmodified voxel cells during grading

Use checkboxes instead of toggle buttons in rail planner UI

Add edge plane snapping
Fix saving of pinned tangents

Pin newly placed nodes so they don't move when future rail segments are added
Snap to targeted bendy edges, so targeting the top of existing rails will put the node at the right vertical height to line up.
Improve suggested grade and direction of new nodes to calculate the suggested grade in isolation, usually giving better results.

Remember rail planner settings

Improve end point tangent calculation
Reduce node snapping distance when long placing

Support pinning of railway segment tangents
Show max curvature and grade while placing segments
Allow overriding direction and grade of placed nodes
Remove V2 segment placer

Allow admins to remove edges