Artificial Intelligence Pathfinding
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Moving monsters ain't easy. The first version of kamikaze let them wander along
the walls, so they could never get into the middle of the playfield.
The second version added some randomization, yet the movements were pretty
predictable.

In kamikaze 0.0.4pre, pathfinding was introduced. It was triggered whenever an
item was uncovered (such as additional life or power), and using a depth-first
flood-fill algorithm calculated in advance the way of each monster to the item.

Features:
- path was recalculated whenever a 'dirty flag' was recognized (such as a tile
  in the path marked as walkable wasn't walkable anymore)
Drawbacks:
- any monster which couldn't reach the item was recalculated in ever round.
- paths were not the shortest ones, and emerging path shortcuts were not
  considered

To solve the problems, the new algorithm comes into the game.
It uses multiple breadth-first iterations, and thus guarantees that whenever
the first path is found, it is the shortest one.
The drawback, that in each iteration many previously examined tiles must be
examined again, is a small one, given that no useless recalculations occur.
(This can be ensured by introducing a list of unreachable tiles for each
monster, linked with a timestamp.)

Possible improvements:
- dynamic updates (possible, but consumes a lot of time)
- lower bounds (punishments) to exclude paths that are short but go nearby
  bombs or enemies

Note that all those optimization only affect CPU usage, not network or I/O
usage, and barely differ in memory requirements.