Fix #137942: Grease Pencil: Make layer operators non-editable for linked data

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

scripts/startup/bl_operators/grease_pencil.py

@@ -32,7 +32,8 @@ class GREASE_PENCIL_OT_relative_layer_mask_add(Operator):
             (obj := context.active_object) is not None and
             obj.is_editable and
             obj.type == 'GREASEPENCIL' and
-            obj.data.layers.active is not None
+            obj.data.layers.active is not None and
+            obj.data.is_editable
         )
 
     def execute(self, context):