Add mineclonia support

Hey I made a fairly simple patch to add mineclonia support.

As you can see there is a node callback for when a node gets hit by an arrow and hence the entity isn't even needed. But for the record setting _mcl_piston_unmovable = true in the entity would make it unmovable for mcl_pistons - this is imo a way better design than with mesecons where you have to depend on the mod and call a function from it but i digress.

I fully understand that you are annoyed about this but maybe this helps to show that it isn't all so bad. I might also add, even though I think you probably did not realize that at the time, that the _hittable_by_projectiles thing that you readily added is a fairly recent addition by voxelibre that was never present in mineclonia, so even if we had changed absolutely nothing this would not have worked.

It is definitely true that we sometimes inadvertently break things even if we try to be careful not to, I suppose this is hardly avoidable if you're trying to clean up an old and huge codebase like ours. However we also try to fix issues caused by this.

Even though I (and other people more so it would seem :p) would be delighted to converse in a more friendly tone, you are nevertheless always welcome to let us know if there are any issues or you need help with anything.