Hump yard stuff

[t0stiman]

In this (unfinished) PR I will be adding some functionality that will help to create hump yards. I'm creating this draft PR because I'm starting to lose track of what I'm making 😅. And some interim feedback wouldn't hurt.

Components:

• Retarders: used in hump yards to slow traincars down. I'm only making the functional part and not the visual; I can't model.
• YardController: this component will automatically send a traincar to the right track by setting the switches. Just like in modern hump yards. I am still working out how exactly I will let players set a destination for a traincar.
• TimedCarSpawner: add this component to a track to spawn a traincar on the track every X seconds. Mostly useful for testing purposes.
• CarDeleter: add this component to a track and every traincar that rolls onto the track is instantly despawned. Also meant for testing.

Track tools:

• Custom switches: the ability to create switches in any shape you like, with any number of branches.
• An option to disable track previews (the blue, red and green gizmos)