Add ActionQueue priority system

This makes effectors nearer to the source of the action (the receptor) update first.

This defines behaviour for this piston circuit: http://i.imgur.com/9Pp2Mzb.png
And defines, that this memory circuit does not work from this direction: http://i.imgur.com/jJn0aFh.png
But it will work when using the switch from the other side: http://i.imgur.com/nvw0oZB.png

Only if two effectors have the same distance, there is nothing we can do about it, behaviour is not defined.
"Distance" is determined by the stack size of recursions in turnon / turnoff.
Priorities are between 0 (lowest) and 1 (highest).

mesecons/actionqueue.lua

@@ -6,15 +6,19 @@ end
 
 -- If add_action with twice the same overwritecheck and same position are called, the first one is overwritten
 -- use overwritecheck nil to never overwrite, but just add the event to the queue
-function mesecon.queue:add_action(pos, func, params, time, overwritecheck)
+-- priority specifies the order actions are executed within one globalstep, highest by default
+-- should be between 0 and 1
+function mesecon.queue:add_action(pos, func, params, time, overwritecheck, priority)
 	-- Create Action Table:
 	time = time or 0 -- time <= 0 --> execute, time > 0 --> wait time until execution
+	priority = priority or 1
 	overwritecheck = overwritecheck or {}
 	action = {	pos=mesecon:tablecopy(pos),
 			func=func,
 			params=mesecon:tablecopy(params),
 			time=time,
-			owcheck=mesecon:tablecopy(overwritecheck)}
+			owcheck=(overwritecheck and mesecon:tablecopy(overwritecheck)) or nil,
+			priority=priority}
 
 	--print(dump(action))
 	-- if not using the queue, (MESECONS_GLOBALSTEP off), just execute the function an we're done
@@ -42,6 +46,18 @@ end
 -- this makes sure that resuming mesecons circuits when restarting minetest works fine
 -- However, even that does not work in some cases, that's why we delay the time the globalsteps
 -- start to be execute by 5 seconds
+local get_highest_priority = function (actions)
+	local highestp = 0, highesti
+	for i, ac in ipairs(actions) do
+		if ac.priority > highestp then
+			highestp = ac.priority
+			highesti = i
+		end
+	end
+
+	return highesti
+end
+
 local m_time = 0
 minetest.register_globalstep(function (dtime)
 	m_time = m_time + dtime
@@ -53,10 +69,14 @@ minetest.register_globalstep(function (dtime)
 		if action.time > 0 then
 			action.time = action.time - dtime
 			table.insert(mesecon.queue.actions, action) -- will be handled another time
-		else -- execute and remove
-			mesecon.queue:execute(action)
 		end
 	end
+
+	while(#actions > 0) do -- execute highest priorities first, until all are executed
+		local hp = get_highest_priority(actions)
+		mesecon.queue:execute(actions[hp])
+		table.remove(actions, hp)
+	end
 end)
 
 function mesecon.queue:execute(action)