Properly set values in subforms

[randm-ch]

Instead of relying on setTimeout which can be flaky for obvious reasons, we now spawn a listener on the updating Signal to set the new value on the subform.

The downside here is that an Owner must be spawned each time we do this (I'm not sure about the overhead?) but we can can also kill again it each time it's done; hopefully this is enough to prevent leaks.

[kitlangton]

I'll take it! Perhaps the experts on the laminar gitter might have some ways of getting around the manual owner, but this seems great for now :)