Some functionality of `Layer`s and `LayerGroup`s can be shared. This is why the `TreeNode` class exists. It encapsulates all the common methods both `Layer`s and `LayerGroup`s should have.
Up until now, the usage of `TreeNode` was not consistent and often the layers would just access the base c-struct directly.
This refactor makes it so that all of the functions affecting only the `TreeNode` are moved to the `TreeNode` class and only implemented there.
For example, renaming a layer is really just renaming the node. Therefor there shouldn't even be a `rename_layer` and `rename_layer_group` function. These were combined into a `rename_node` function.
Pull Request: https://projects.blender.org/blender/blender/pulls/111177
b14a0ee57e