give objecs a specific id
I don't think the "sort object list" property will be implemented any time soon (see http://bugs.openttd.org/task/4961). So in order to avoid breaking compatibility again when adding a new object somewhere in the middle of the list, objects should get a fixed ID, with some unused IDs in between.
Some groups of objects don't need to have unused IDs inbetween (like the activators), only at the end of the group. Leaving room for 16 activators in total should be enough. The rest can be figured out later.