Grease Pencil layer groups are drawn in the layer tree using
`AbstractTreeViewItem`. The problem is that the items can't
store their expand/collapse state. This meant that switching
e.g. the tabs in the properties editor would reset the collapsed
state of layer group items.
This PR uses the `GP_LAYER_TREE_NODE_EXPANDED` flag to store
the expand/collapse state. The tree view then reads from the flag to
expand/collapse the items.
This also adds an RNA property to layer groups `is_expanded`.
Allows to check from python if a group is expanded in the UI.
Pull Request: https://projects.blender.org/blender/blender/pulls/131159
e5bdfd533b