Adds a new UI template to view the current layer tree of the active Grease Pencil object.
This UI tree view implements the following features (for now):
- Displaying all the layers with their names and highlighting the active layer.
- Changing the active layer by clicking on an item.
- Adding new layers (using a new operator).
- Removing the active layer (using a new operator).
- Renaming a layer.
Pull Request: https://projects.blender.org/blender/blender/pulls/109197
b3f3b41d09