New drag type `WM_DRAG_GREASE_PENCIL_GROUP` introduced for layer group nodes.
Replaced layer struct with `GreasePencilLayerTreeNode` in `wmDragGreasePencilLayer`
so group node can be stored while dragging in `wmdrag->poin`.
Using `can_drop()`, drop operation of layer group node returns `false`
if the target node is its child.
Part of #121390.
Pull Request: https://projects.blender.org/blender/blender/pulls/121654
9b5f0cb8a2