Merge pc.GraphNode into pc.Entity

History
The engine has a layered architecture:

Layer 4 (highest): Framework (Entity, Component, Application)
Layer 3: Scene (GraphNode and related scene classes)
Layer 2: Graphics (low-level rendering layer)
Layer 1 (lowest): Core

Any given layer should be usable independent of any higher level.

Proposal
pc.GraphNode (from the scene layer) should be merged into pc.Entity. One option is to move pc.Entity, pc.Component (just component base class) and pc.Application to the core layer (maintaining the one way dependencies across layers).

Layer 4 (highest): (Components)
Layer 3: Scene
Layer 2: Graphics
Layer 1 (lowest): Core (Entity, Component, Application)

Rationale

• The distinction between entities and graph nodes is needless and confusing.
• Simplifies the API (and makes the API reference easier to follow).
• Loaded models have hierarchies built from graph nodes rather than entities. This means it is not possible to directly add components to nodes in a model hierarchy (although you can add a pc.Entity as a child of a pc.GraphNode).
• This change would pave the way for displaying a model's hierarchy in the Hierarchy panel of the Editor.

Problems

1. There is code in the engine somewhere (or possibly the Editor) related to picking. A picked meshinstance is attached to a graph node and the node's anscestors are walked to find the parent entity. Some thought needs to be given to how we handle picking if this change is implemented.
2. It is difficult to make this change without creating circular dependencies in the codebase. The pc.Entity class has a dependency on pc.Application which has a dependency on many classes in the scene layer.

Backwards Compatibility
pc.GraphNode should be aliased to pc.Entity.

Start by tracing pc.Entity, pc.GraphNode, pc.Application, and the scene-layer dependencies to understand where circular dependencies arise. Then inspect the engine or Editor picking path, especially how meshinstance ancestors are walked to find a parent entity. Done means GraphNode is merged into Entity, backward compatibility is preserved through an alias, and picking and layer dependencies still work.