PortalStructure now contains information about the gate's physical structure, such as the location of border blocks, the location of entrances, the gate type and the button location.
PortalActivator is responsible for activating/de-activating portals, destination toggling and getting information about available destinations.
PortalOpener is responsible for opening/closing a portal. It's also the place to go for checking if the portal is open for a given player.
Comments of the Portal class have been improved, but the comments of the three new classes need fixing.
Adds all chunk unloading to a queue
Adds a thread which unloads chunks
Updates chunk unload requests such that a chunk won't be unloaded twice,
and an old unloading request cannot unload a chunk too soon
Because of the teleport change, end gateways work to teleport player,
and end gateways work to the end for vehicles, but vehicles cannot teleport back from the end
