Added the sonar pulse to reveal sub marines

As described in #4729 this was never implemented. Balancing wise I decided to make this a localized but multi-shot event similar to the spy plane. A good test case is http://resource.openra.net/maps/495/ which has creeps with an infiltratable sub pen and a submarine next to it plus soldiers that can be used as a disguise against the pillboxes.
Immediate powers don't display beacons, so this is redundant (Airstrike and Nuke are the only other powers that implement this).