Layout improvements to Graph Editor (#2775)

This changelist improves the Layout engine in the Graph Editor, organizing its logic into a new Layout class, and aligning its approach with the Sugiyama layered graph drawing algorithm (https://en.wikipedia.org/wiki/Layered_graph_drawing).

Additionally, layout is now performed over two frames, allowing the true rendered dimensions of each node to be used in final node placement.

Two specific issues addressed by this changelist are #1414 and #2432.

Author: jstone-lucasfilm