Control leaf decay with sneak.

As requested by @paramat.
Leaves are preserved by default and only decay when using place + sneak.

mods/default/functions.lua

@@ -257,10 +257,12 @@ minetest.register_globalstep(function(dtime)
 end)
 
 default.after_place_leaves = function(pos, placer, itemstack, pointed_thing)
+	if placer and not placer:get_player_control().sneak then
 		local node = minetest.get_node(pos)
 		node.param2 = 1
 		minetest.set_node(pos, node)
 	end
+end
 
 minetest.register_abm({
 	nodenames = {"group:leafdecay"},