Show nodes as mini layers in the Layers panel

The layer panel is mainly designed to show layers, but because non-layer nodes can also be included at the bottom of a layer stack, or within the middle of it (causing the stack to "stair-step" within the graph), it becomes unclear how the data is sourced and transformed if the layer panel lacks that information. Those nodes should be added to the layer panel, but made skinnier and without thumbnails, in order to show a full picture of what's going on in the graph even without needing to look at the graph.

Compared to the more complex Outliner panel, the Layers panel only allows expansion of layers, not node secondary inputs, and doesn't show the data flow wires or other complexities that the Outliner panel shows in order to provide a near 1:1 mapping to the graph.