In preparation of https://projects.blender.org/blender/blender/pulls/112535, we need to make sure all layer tree manipulations are in functions on the `GreasePencil` struct so we can keep the `CustomData` for layers in sync with the layer tree.
This PR makes sure that all the functions that change the layer tree in any way are `protected` on the `LayerGroup` class. The `GreasePencil` struct is declared `friend` with `LayerGroup`.
We also cleanup and remove a lot of code duplication between layers and layer groups and try to write functions in a generic way for `TreeNode` (both layers and layer groups are tree nodes).
Pull Request: https://projects.blender.org/blender/blender/pulls/112837
16e195a111