updated underlay plating to work with new construction system (almost exact copypaste, todo: merge with overlayplating somehow), updated underlay plating material, plating does not seem to get destroyed when a girder is reverted to a buildpoint

Signed-off-by: Miles Whiticker <miles.whiticker@gmail.com>
fixed up some last stuff with buildpoints/mounted structures on girders, removed a bunch of old unused code

Signed-off-by: Miles Whiticker <miles.whiticker@gmail.com>