Allow Nesting LatticeGraphs as Sub-Graphs

[Kleptine]

Low priority, but this would be nice for larger composition. Ie. I wrote a little group of nodes to spawn an effect and give some resources on death of an enemy. Right now I have to copy-paste this to each enemy's graph:

It'd be nice to be able to embed this in several graphs more easily.

Something I realized thinking about this:

• We can just make all Graphs nestable. We don't need to have a distinction between "Top-level" and "Macro" graphs, the way blueprints does. Kind of nice.
• Props for nested graphs (ie. showing inspectors on the nodes) is the same exact problem as storing bindings for scene objects. Ie. If we build one of these systems, we'll have the other.

As a plus, if we use a binding system, we'll be able to show properties of a graph in the LatticeExecutorAuthoring inspector for all graphs.

[Kleptine]

Recently I added the ability for entities to run multiple LatticeGraphs on them. This was a simple change, but actually accomplishes much of what SubGraphs would be useful for! For example, you can write a "PlayVFXOnDeath" LatticeGraph and attach it to an entity.

Multiple LatticeGraphs is also our path to inheritance-like functionality. Simply add another lattice graph for the child's extra logic.

What you can't do, though, is send inputs to the separate graphs, or compose several of them together (ie. several PlayVFXOnDeath graphs on a single entity). So this feature would still be very useful for wrapping up behavior that needs more complex composition.