MapRenderer: Try harder to place enemy groups

* Increase the number of random tries from 3 to 5 allowed misses per enemy
* In case we still have failure, try a deterministic method, by iterating
  over all tiles in the given rectangle and place at the first free spot.
  This is not random, but should occur only on rare occasions.
* Warn if we still cannot place all enemies.

Fixes #742.

Signed-off-by: Stefan Beller <stefanbeller@googlemail.com>
EnemyManager: Warn if enemy has empty type

This was warning was removed in the previous commit. There is was only
be checked for enemy groups however. Now check for any enemies.

Signed-off-by: Stefan Beller <stefanbeller@googlemail.com>
Merge pull request #794 from stefanbeller/master

Fix #742