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.