Replace legacy layer types with nodes

[Keavon]

Task: Switch LayerDataType struct values to nodes

#[derive(Debug, Clone, PartialEq, Deserialize, Serialize)]
/// Represents different types of layers.
pub enum LayerDataType {
	/// A layer that wraps a [FolderLayer] struct.
	Folder(FolderLayer),
	/// A layer that wraps a [ShapeLayer] struct.
	Shape(ShapeLayer),
	/// A layer that wraps a [TextLayer] struct.
	Text(TextLayer),
	/// A layer that wraps an [ImageLayer] struct.
	Image(ImageLayer),
        Imaginate(ImaginateLayer),
}

FolderLayer, ShapeLayer, TextLayer, and ImageLayer live in /graphene. Each of these structs (saving FolderLayer for another issue) should be converted into nodes using the new node system in /node-graph.

---

Questions for @TrueDoctor:

• How does this approach relate to the separation between "document graph" and "proto graph"?
• Can we continue to reference the other data structures defined in /graphene which are used by those three (four) structs once they are ported to nodes in /node-graph?
• Where do these new vector-editing-specific nodes go in the node graph folder structure?
• Any other things to be aware of?

[TrueDoctor]

These would be document graph nodes so we are kinda blocked by my work on document nodes there

[Keavon]

@TrueDoctor Ok, thanks for clarifying that. Would you be able to meet with @0HyperCube and me soon to design that?