Defense Tower doesn't damage bots or buildings

[melex750]

Affected objects are defined in particle.cpp.
Full damage is applied to the first object found within a radius at target position.

Maybe the object closest to target position should take damage,
or possibly all objects in the target radius could be damaged.

Maybe reduced damage for friendly objects.

[ghost]

Actually all damage rules in the game require some rethink and rewriting. Also using particles to this process seems unstable as issue #910 states. Possibly we should discuss all aspects of the damage rules somewhere before we'll fix this one issue.