Ports overmap events from Baystation12

- Move overmap defines to _defines folder.  Rename old file to turfs.dm since that is what it contains.
- Definition of overmap event objects and the overmap event handler.
- Upgrades to SSevents and SSskybox to tie in the overmap events.
- Enhancement to /datum/event itself to support affecting_z and victim ship.
- Upgrade to the five event types used on the overmap to support new vars.
- Upgrade to dust and meteor spawning code to support targeting z-levels.

code/_helpers/game.dm

@@ -26,6 +26,15 @@
 		max_z = max(z, max_z)
 	return max_z
 
+/proc/living_observers_present(var/list/zlevels)
+	if(LAZYLEN(zlevels))
+		for(var/mob/M in player_list) //if a tree ticks on the empty zlevel does it really tick
+			if(M.stat != DEAD) //(no it doesn't)
+				var/turf/T = get_turf(M)
+				if(T && (T.z in zlevels))
+					return TRUE
+	return FALSE
+
 /proc/get_area(atom/A)
 	if(isarea(A))
 		return A

code/_helpers/turfs.dm

@@ -33,6 +33,20 @@
 		available_turfs = start_turfs
 	return pick(available_turfs)
 
+// Picks a turf that is clearance tiles away from the map edge given by dir, on z-level Z
+/proc/pick_random_edge_turf(var/dir, var/Z, var/clearance = TRANSITIONEDGE + 1)
+	if(!dir)
+		return
+	switch(dir)
+		if(NORTH)
+			return locate(rand(clearance, world.maxx - clearance), world.maxy - clearance, Z)
+		if(SOUTH)
+			return locate(rand(clearance, world.maxx - clearance), clearance, Z)
+		if(EAST)
+			return locate(world.maxx - clearance, rand(clearance, world.maxy - clearance), Z)
+		if(WEST)
+			return locate(clearance, rand(clearance, world.maxy - clearance), Z)
+
 /proc/is_below_sound_pressure(var/turf/T)
 	var/datum/gas_mixture/environment = T ? T.return_air() : null
 	var/pressure =  environment ? environment.return_pressure() : 0