GPv3: Rename layer domain to just "Layer"

We had a short discussion on this change. "Grease Pencil Layer" is
just a bit too long, especially in the UI. Even though "Layer" might be
ambiguous, it shouldn't be in the context of geometry nodes. There are
currently no other "Layers" and if there were, using the same domain
name could be fine (just like we reuse the point domain for e.g. vertices
in meshes and control points in curves).

This also renames the internal enum to `ATTR_DOMAIN_LAYER`

Pull Request: https://projects.blender.org/blender/blender/pulls/113589

source/blender/makesdna/DNA_grease_pencil_types.h

@@ -403,7 +403,7 @@ typedef struct GreasePencil {
   GreasePencilLayerTreeGroup *root_group_ptr;
 
   /**
-   * All attributes stored on the grease pencil layers (#ATTR_DOMAIN_GREASE_PENCIL_LAYER).
+   * All attributes stored on the grease pencil layers (#ATTR_DOMAIN_LAYER).
    */
   CustomData layers_data;
   /**