Add Current Graphics Keys to GraphicsComponent

[eonarheim]

Context

Currently for a given Actor's GraphicsComponent, it's difficult to know what the currently shown graphics are by name. It is useful to know the name's of the graphics shown in all layers or a specific layer

To iterate through all the graphics layers

for (const layer of graphicsComponent.layers.get()) {
  ...
}

Proposal

1. Add a new getter to GraphicsComponent.ts called currentKeys that will return the names of the graphics shown in all layers
2. Add a new getter to GraphicsLayer called currentKeys that will the names of the graphics shown in this layer
3. If a graphic doesn't have a name show "anonymous" as the name
4. Additional internal data structure may be needed to keep track of the shown graphics per layer
5. New unit tests are required

[ignoreintuition]

You want me to take a stab at this one?

[eonarheim]

@ignoreintuition all yours!