Previously, only one level of panels was supported. Now, they can be nested arbitrarily.
Panels still have to come at the bottom though.
The panel color used to be just the node color darkened a bit. Now it uses the
`TH_PANEL_SUB_BACK` theme setting which is also used by panels in other places
in Blender. However, the contrast of that is a bit weaker than what we had in nodes before.
Therefore I increased the contrast a bit.
Pull Request: https://projects.blender.org/blender/blender/pulls/128886
7dc630069b