[x] write / read individual entities. reading over an entity with a Some PropagatedDescriptorOpt will preserve its value instead of taking the value from the file.
[x] ui to clear propagation targets
[x] ctrl+p to propagate selected
[x] propagator check box in entity search
[x] cltr+shift+drag => duplicate without setting PropagationSourceOpt
[x] ctrl+shift+v => paste without setting PropagationSourceOpt
We have this mostly implemented in https://github.com/bryanedds/Nu/tree/propagation, but there's a couple more things -