Spaceports

I added the new Structure Spaceport to transfer Items between Planets.
Thanks to this the Planets can actually be connected and aren’t ‘separate games’ anymore.
This is a very basic, first implementation and it works this way:

• Spaceports of different Planets can be linked
• Items placed into the ‘from’ Spaceport are available at the ‘to’ Spaceport
• Items can be placed into and taken from the Spaceport