PropertySupport: allow descriptor-less properties

[#48361799]
PropertySupport: before/afterSetCallbacks

[#48361799]
MD2D: ignore nested invalidating changes

Almost surely this is OK, right?
MD2D: invalidate after property assignment

[#48361799]
timeStep change no longer needs to reset graph
Rebuild Lab when signal-generator model is changed

[#48361799]
Makefile: improve handling of new interactive dirs
signal-generator model and test interactive

[#48361799]

This is roughly the minimal model that can be graphed.