lp:~widelands-dev/widelands/attack_fix

Small fix to AI regarding the atttacks. Two points:

- AI now test visibility of a building it is going to attack - this was in fact a bug - though not visible to human player - AI was trying to attack but 'engine' had not allowed such attack because a site was not visible to AI player. Just now I found out what was going on
- Add some randomness to number of attack soldiers - ths will affect situation when there is more then 10 available attack soldiers.