abstract most inventory handling code into a homedecor registration function

drawtype and param related defaults are being handled as well
for now we still leave the locked-node generation to locked.lua, due to its furnace-like node support

example:
homedecor.register("penbox", {
	<common defs>
	infotext=S("Penbox"),
	inventory = {
		size=8,
		formspec=…
		lockable=true -- will be supported later
	},
})

homedecor/init.lua

@@ -222,8 +222,9 @@ function homedecor.find_ceiling(itemstack, placer, pointed_thing)
 	return isceiling, pos
 end
 
--- load various other components
+dofile(homedecor.modpath.."/registration_handler.lua")
 
+-- load various other components
 dofile(homedecor.modpath.."/misc-nodes.lua")					-- the catch-all for all misc nodes
 dofile(homedecor.modpath.."/tables.lua")
 dofile(homedecor.modpath.."/electronics.lua")