support SVG blur effect on subdiagrams

[kostmo]

This is especially useful for drop shadows.

See SVG spec: https://developer.mozilla.org/en-US/docs/Web/SVG/Element/feGaussianBlur

[byorgey]

This seems like something that could certainly be added, but since it's so specific to SVG I think the place for it to go is in the diagrams-svg backend rather than in diagrams-core. Could you open a ticket over on the diagrams-svg issue tracker?