Added LineBuildNode trait to filter which structure(s) a LineBuild actor can be attached to.

D2k walls now attach to turrets.
This has been requested and serves as an example for this trait pair.

In this example the Sandbag `SBAG` will LineBuild onto itself and the `SOMEDEF` actor definition.
You can see how this can be used for very generic or specific cases.

```yaml
SBAG:
	LineBuild:
		NodeTypes: sbag, chicken
	LineBuildNode:
		Types: sbag

SOMEDEF:
	LineBuildNode:
		Types: chicken
```