UI: Support drag & drop reordering of collections

This adds initial support for reordering collections from the Outliner
using drag & drop.
Although drag & drop support is limited to collections for now, this
lays most foundations for general drag & drop reordering support in the
Outliner. There are some design questions to be answered though:
* Would reordering of other data types (like objects) be a purely visual change or would it affect the order in which they are stored? (Would that make a difference for the user?)
* Should/can we allow mixing of different data types? (e.g. mixing render layers with objects)
* How could we realize this technically?

Notes:
* "Sort Alphabetically" has to be disabled to use this ("View" menu).
* Reordering only works with collections on the same hierarchy level.
* Added some visual feedback that should work quite well, it's by far not a final design though: {F493806}
* Modified collection orders are stored in .blends.
* Reordering can be undone.
* Did minor cleanups here and there.

1 files changed, 1 insertions, 1 deletions

diff --git a/source/blender/blenkernel/BKE_collection.h b/source/blender/blenkernel/BKE_collection.h
index 4d5de723dce..644bcef6226 100644
--- a/source/blender/blenkernel/BKE_collection.h
+++ b/source/blender/blenkernel/BKE_collection.h
@@ -44,7 +44,7 @@ struct Scene;
 
 struct SceneCollection *BKE_collection_add(struct Scene *scene, struct SceneCollection *sc_parent, const char *name);
 bool BKE_collection_remove(struct Scene *scene, struct SceneCollection *sc);
-struct SceneCollection *BKE_collection_master(struct Scene *scene);
+struct SceneCollection *BKE_collection_master(const struct Scene *scene);
 void BKE_collection_master_free(struct Scene *scene);
 void BKE_collection_object_add(struct Scene *scene, struct SceneCollection *sc, struct Object *object);
 void BKE_collection_object_add_from(struct Scene *scene, struct Object *ob_src, struct Object *ob_dst);