(svn r12913) -Add: ability to backup and restore a player's economic data and data for a vehicle (or chain of vehicles)

Autoreplace uses this with the following benefits:
  -Mass autoreplace (the button in the depot window) will now estimate costs correctly
  -Autoreplace now either replaces correctly or manages to keep the original vehicle (no more broken trains)
  Thanks to Ammler for testing this

src/player_base.h

@@ -73,6 +73,18 @@ struct Player {
 	uint16 num_engines[TOTAL_NUM_ENGINES]; ///< caches the number of engines of each type the player owns (no need to save this)
 };
 
+struct PlayerMoneyBackup {
+private:
+	Money backup_yearly_expenses[EXPENSES_END];
+	PlayerEconomyEntry backup_cur_economy;
+	Player *p;
+
+public:
+	PlayerMoneyBackup(Player *player);
+
+	void Restore();
+};
+
 extern Player _players[MAX_PLAYERS];
 #define FOR_ALL_PLAYERS(p) for (p = _players; p != endof(_players); p++)