Code review comment for lp:~widelands-dev/widelands/expedition_portspace_indicator

OK, now InteractiveBase receives a notification instead when a portspace is found. Since there are fairly many reasons why a port space can become unavailable again (ship sails on, port is constructed, ship is sunk…), the IBase takes care of cleaning up the set itself now when think()ing. This fixes the bug that the indicator won´t go away on portconstruction.
Also added saving support for the indicators so they are also shown for ships that already are in PortSpaceFound state when the game is loaded.