scoped proagators

"Scoped propagators" are formed of a scope and a propagator which often looks like this: click { text: "foo" }

The scope sets the events that cause propagation, such as clicks, ticks, or shape changes (not adding a scope will default to shape changes).

The propagator is a JS object (or function which returns one) that is applied to the shape.

Notes:

shapes are passed both from and to shapes.
Shapes are flattened before being passed to the propagator, and unpacked on the other side. So properties live alongside the x, y, and rotation values (e.g. { x: 100, y: 100, text: "foo" }).

Current Issues (probably should be fixed before putting out a demo):

cycles of change propagators cause infinite recursion.
geo scopes are currently fired for any shape change, this should be localised to spatially local changes.