Cleans up train latching/unlatching

Also adds a verb for turning off train engines.
Merge pull request #5318 from Loganbacca/trainmerge

New vehicle loading code
Merge remote-tracking branch 'upstream/dev' into train-cleanup

Resolved merge conflicts, and added some more user feedback to attach/unattach procs.

Conflicts:
	code/modules/vehicles/cargo_train.dm
Merge pull request #5320 from mwerezak/train-cleanup

Cleans up train latching/unlatching
Verb cleanup

- Removes (temporary) climb down verb in favour of clicking on the train to disembark
- Cleans up some outdated unload code