Hide changes to buildings in the fog-of-war.

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 r15508.

As a temporary adjustment, territories do not explore the map anymore when their borders change. See #2709.

Fixes #599