Direct cargo connections

[LordIdra]

Allows cargo blocks to be directly connected to each other

The routing logic was already there and did not modifying. So the only thing required to make this work in the end was the visual component, and also making sure that the right events were being fired when two cargo blocks were connected. I did this with a slight hack, which is to make cargo blocks entity holders and then add an entity to one of the cargo blocks whenever two cargo blocks are directly connected. This also require extra logic to keep the displays in sync. Whenever a display is created, the PylonCargoConnectEvent is fired, and when a display is destroyed the PylonCargoDisconnectEvent is fired.