To achieve this, mirage entities are created per player, to replace the
real entities when these ones fall into the fog-of-war. These mirage
entities are created on-the-fly, and destroyed when they get back in
sight.
This depends heavily on the VisibilityChanged message added in
2174eaaeee.
As a temporary adjustment, territories do not explore the map anymore
when their borders change. See #2709.
Fixes#599
This was SVN commit r15612.
