Get UI ready for using drag & drop to nest a collection into another one

Nothing happen yet when it's supposed to insert the collection into
another one, that part will be handled by @dfelinto.

See gif for demo of how it works UI wise: {F500337}

Also fixed off-by-one error in utility function.

1 files changed, 9 insertions, 3 deletions

diff --git a/source/blender/editors/space_outliner/outliner_tree.c b/source/blender/editors/space_outliner/outliner_tree.c
index 8a024e0cc82..0cf76a671a4 100644
--- a/source/blender/editors/space_outliner/outliner_tree.c
+++ b/source/blender/editors/space_outliner/outliner_tree.c
@@ -1287,13 +1287,19 @@ static void outliner_add_orphaned_datablocks(Main *mainvar, SpaceOops *soops)
 	}
 }
 
-static void outliner_collections_reorder(const Scene *scene, TreeElement *insert_element, TreeElement *insert_after)
+static void outliner_collections_reorder(const Scene *scene, TreeElement *insert_element, TreeElement *insert_handle,
+                                         TreeElementInsertType action)
 {
 	SceneLayer *sl = BKE_scene_layer_render_active(scene);
 	LayerCollection *insert_coll = insert_element->directdata;
-	LayerCollection *insert_after_coll = insert_after ? insert_after->directdata : NULL;
+	LayerCollection *insert_handle_coll = insert_handle ? insert_handle->directdata : NULL;
 
-	BKE_layer_collection_reinsert_after(scene, sl, insert_coll, insert_after_coll);
+	if (action == TE_INSERT_AFTER) {
+		BKE_layer_collection_reinsert_after(scene, sl, insert_coll, insert_handle_coll);
+	}
+	else if (action == TE_INSERT_INTO) {
+		BKE_layer_collection_reinsert_into(insert_coll, insert_handle_coll);
+	}
 }
 
 static void outliner_add_layer_collections_recursive(SpaceOops *soops, ListBase *tree, Scene *scene,