Add support for layers

[magjac]

Graphviz supports using layers (overlays).

It would be nice if the user could select which layer to show, e.g. through a new .layer() method. If animated transitions are used, the graph could do an animated transition from the current layer to the new one.

It would also be possible to implement some kind of movie mode where the graph could transition through all the layers, one at a time.

Note also bug #163.

Some more background info can be found at:

https://forum.graphviz.org/t/how-to-share-svg-files/101
https://forum.graphviz.org/t/getting-layers-to-work-with-svg/107

https://forum.graphviz.org/t/how-to-share-svg-files/101
https://forum.graphviz.org/t/getting-layers-to-work-with-svg/107