For linked grease pencil data, layer operators are accessible right
now. Grey out them in UI by adjusting poll functions. Also disabled
the individual layer row of tree-view.`
See images in PR description
Pull Request: https://projects.blender.org/blender/blender/pulls/137946
Adds a new python operator to Grease Pencil for adding the layer
above or below as a masking layers to the active layer.
Creates a new file for Grease Pencil python operators in
`scripts/startup/bl_operators/grease_pencil.py`.
Pull Request: https://projects.blender.org/blender/blender/pulls/134401