Exposing z-index of objects

[ugurdogrusoz]

Cytoscape.js core supports a depth for each node/edge. This is especially useful for simple vs compound node overlaps (e.g. a macromolecule on the border of a compartment). In fact, simple-simple node overlaps are forbidden in SBGN but users might still want to draw them overlapping (temporarily or otherwise) and might want to change their drawing order.