Added event type "unit placed"

[ln-zookeeper]

This is intended to allow event-driven abilities and other modifications to immediately take action when any unit appears on the map.

Still needs some more testing in context of replays and multiplayer.

[gfgtdf]

You need to be carteful when calling game_events->fire(...) becasue events can basically change everything. For example it can kill other units (like killing the attacking unit in an attack event) which casues unexpected behaviour or even crashes when the c++ code assumes that these units are still there.

[ln-zookeeper]

I think it's safe; I've tested an event which kills all created units (except leaders) and the only problem it caused was an infinite AI recruitment loop. I'll try to see if there's a way to avoid that.

I also noticed some kind of a bug involving plague which I'll need to figure out.