Graph node z order is inconsistent

[ogallagher]

Depending on the order that they’re added to the graph, edges can sometimes display in front of cells instead of always being behind them (see demo video).

This could be fixed by specializing the cartesian graph and its canvas to group cells and edges separately, or by modifying those base classes directly to do so. I plan to do the latter, updating Graph and PannableCanvas to separate edges and cells into distinct groups, with a customizable z order.