Show enemy ships if player loses the match.

As requested by a user a long time ago, oponnent's ships are nicely
displayed at the end of a battle if the player loses a game. This way
the defeated player can study the adversary's strategy and plan ahead
for future matches.

Currently the whole feature is a work in progress. It works on single
player mode only and has no entry at the settings interface, thus
working by default with no possibility of turning it off.

Next commits will add support for multiplayer games and will also make
it possible to turn this feature on/off through the settings interface.