Leafdecay: Node timer based implementation, API

This implements a node-timer based leafdecay mechanism, and exposes
an API to use it in mods.

The API is documented in game_api.txt.

`default.register_leafdecay(leafdecaydef)`

`leafdecaydef` is a table, with following members:
{
	trunks = { "default:tree"}, -- nodes considered trunks
	leaves = { "default:leaves", "default:apple"}, -- nodes considered leaves
	radius = 3, -- activates leafdecay this far from the trunk
}

The algorithm will drop `leaves` items in the area if no `trunks` are found
in the `trunk_radius` sized area around the position of the leaf. If a node
listed in `leaves` has a group `leafdecay_drop > 0`, then the item is dropped,
otherwise the item is removed but not dropped.

The algorithm also implements a value `default.leafdecay_speed` (default
15) which can be modified to increase or decrease of the leaf decay. The
algorithm will vary the actual speed a bit to introduce randomness.

Leaf decay is randomized by 0.1 seconds to reduce the chance that
decay happens many times on the same second interval. This requires
nodetimer_interval to be set to values lower than 1.0 to have an
effect.

The leaves will decay between 2 and 10 seconds after digging the trunk,
and happen at non-integer second intervals.

-- The API was added by sofar.

mods/default/nodes.lua

@@ -2072,3 +2072,57 @@ minetest.register_node("default:cloud", {
 	sounds = default.node_sound_defaults(),
 	groups = {not_in_creative_inventory = 1},
 })
+
+--
+-- register trees for leafdecay
+--
+
+if minetest.get_mapgen_setting("mg_name") == "v6" then
+	default.register_leafdecay({
+		trunks = {"default:tree"},
+		leaves = {"default:apple", "default:leaves"},
+		radius = 2,
+	})
+
+	default.register_leafdecay({
+		trunks = {"default:jungletree"},
+		leaves = {"default:jungleleaves"},
+		radius = 3,
+	})
+
+	default.register_leafdecay({
+		trunks = {"default:pine_tree"},
+		leaves = {"default:pine_needles"},
+		radius = 3,
+	})
+else
+	default.register_leafdecay({
+		trunks = {"default:tree"},
+		leaves = {"default:apple", "default:leaves"},
+		radius = 3,
+	})
+
+	default.register_leafdecay({
+		trunks = {"default:jungletree"},
+		leaves = {"default:jungleleaves"},
+		radius = 2,
+	})
+
+	default.register_leafdecay({
+		trunks = {"default:pine_tree"},
+		leaves = {"default:pine_needles"},
+		radius = 2,
+	})
+end
+
+default.register_leafdecay({
+	trunks = {"default:acacia_tree"},
+	leaves = {"default:acacia_leaves"},
+	radius = 2,
+})
+
+default.register_leafdecay({
+	trunks = {"default:aspen_tree"},
+	leaves = {"default:aspen_leaves"},
+	radius = 3,
+})