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, 31 insertions, 2 deletions

diff --git a/source/blender/blenkernel/intern/collection.c b/source/blender/blenkernel/intern/collection.c
index 100cbf2dd45..24f97c00afb 100644
--- a/source/blender/blenkernel/intern/collection.c
+++ b/source/blender/blenkernel/intern/collection.c
@@ -44,6 +44,9 @@
 
 #include "MEM_guardedalloc.h"
 
+bool collection_remlink(SceneCollection *, SceneCollection *);
+bool collection_insert_after(SceneCollection *, SceneCollection *, SceneCollection *);
+
 /**
  * Add a collection to a collection ListBase and syncronize all render layers
  * The ListBase is NULL when the collection is to be added to the master collection
@@ -95,7 +98,7 @@ static void collection_free(SceneCollection *sc)
  * Unlink the collection recursively
  * return true if unlinked
  */
-static bool collection_remlink(SceneCollection *sc_parent, SceneCollection *sc_gone)
+bool collection_remlink(SceneCollection *sc_parent, SceneCollection *sc_gone)
 {
 	for (SceneCollection *sc = sc_parent->scene_collections.first; sc; sc = sc->next)
 	{
@@ -174,9 +177,35 @@ bool BKE_collection_remove(Scene *scene, SceneCollection *sc)
 }
 
 /**
+ * Lookup the parent listbase of \a sc_insert_after and insert \a sc_insert after it.
+ * \param sc_after: If this is NULL, \a sc_insert will be inserted as first collection in \a parent.
+ */
+bool collection_insert_after(
+        SceneCollection *parent, SceneCollection *sc_insert, SceneCollection *sc_after)
+{
+	if (sc_after == NULL) {
+		BLI_addhead(&parent->scene_collections, sc_insert);
+		return true;
+	}
+
+	for (SceneCollection *sc = parent->scene_collections.first; sc; sc = sc->next) {
+		if (sc == sc_after) {
+			BLI_insertlinkafter(&parent->scene_collections, sc_after, sc_insert);
+			return true;
+		}
+
+		if (collection_insert_after(sc, sc_insert, sc_after)) {
+			return true;
+		}
+	}
+
+	return false;
+}
+
+/**
  * Returns the master collection
  */
-SceneCollection *BKE_collection_master(Scene *scene)
+SceneCollection *BKE_collection_master(const Scene *scene)
 {
 	return scene->collection;
 }