This algorithm - to be done:
2. Add n-level dependencies control. How? I don't know
3. Check for boxes depending on movement from never-reachable areas.

This algorithm - done:
* Add checking for boxes dependant on blocked ones.
  * It's not true that a box depending on a blocked SHOULD BE blocked. It may
  also depend on some other to become free. The amount of dependencies must
  be checked.

Other algorithm - endpoints:
We don't treat box moves as moves to next cell, but to moves until an endpoint.
This way, huge maps with many free space will be solved much faster.