Split wires into their own mod, introduce autoconnect hooks

The update_autoconnect function had to be abstracted away from the
default wires, any kind of wire can now register autoconnect hooks,
which should make having multiple different wire types much easier.

mesecons_mvps, mesecons_receiver and mesecons_random made use of
update_autoconnect, their code was also adapted. This also fixes a
receiver bug: If a receiver was placed with a onstate receptor next
to it (but not the wall lever / button that caused the receiver to
appear) the receiver didn't turn on in the past.

Also move documentation for mesecon wire into mesecons_wire.

mesecons/util.lua

@@ -273,3 +273,34 @@ mesecon.forceloaded_blocks = mesecon.file2table("mesecon_forceloaded")
 minetest.register_on_shutdown(function()
 	mesecon.table2file("mesecon_forceloaded", mesecon.forceloaded_blocks)
 end)
+
+-- Autoconnect Hooks
+-- Nodes like conductors may change their appearance and their connection rules
+-- right after being placed or after being dug, e.g. the default wires use this
+-- to automatically connect to linking nodes after placement.
+-- After placement, the update function will be executed immediately so that the
+-- possibly changed rules can be taken into account when recalculating the circuit.
+-- After digging, the update function will be queued and executed after
+-- recalculating the circuit. The update function must take care of updating the
+-- node at the given position itself, but also all of the other nodes the given
+-- position may have (had) a linking connection to.
+mesecon.autoconnect_hooks = {}
+
+-- name: A unique name for the hook, e.g. "foowire". Used to name the actionqueue function.
+-- fct: The update function with parameters function(pos, node)
+function mesecon.register_autoconnect_hook(name, fct)
+	mesecon.autoconnect_hooks[name] = fct
+	mesecon.queue:add_function("autoconnect_hook_"..name, fct)
+end
+
+function mesecon.execute_autoconnect_hooks_now(pos, node)
+	for _, fct in pairs(mesecon.autoconnect_hooks) do
+		fct(pos, node)
+	end
+end
+
+function mesecon.execute_autoconnect_hooks_queue(pos, node)
+	for name in pairs(mesecon.autoconnect_hooks) do
+		mesecon.queue:add_action(pos, "autoconnect_hook_"..name, {node})
+	end
+end