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, 17 insertions, 1 deletions

diff --git a/source/blender/editors/space_outliner/outliner_intern.h b/source/blender/editors/space_outliner/outliner_intern.h
index 4685744c570..6a4d53449e9 100644
--- a/source/blender/editors/space_outliner/outliner_intern.h
+++ b/source/blender/editors/space_outliner/outliner_intern.h
@@ -37,6 +37,7 @@
 /* internal exports only */
 
 struct wmOperatorType;
+struct TreeElement;
 struct TreeStoreElem;
 struct bContext;
 struct Scene;
@@ -46,6 +47,13 @@ struct Object;
 struct bPoseChannel;
 struct EditBone;
 
+/**
+ * Callback type for reinserting elements at a different position, used to allow user customizable element order.
+ * Passing scene right now, may be better to allow some custom data.
+ */
+typedef void (*TreeElementReinsertFunc)(const struct Scene *scene, struct TreeElement *insert_element,
+                                        struct TreeElement *insert_after);
+
 typedef struct TreeElement {
 	struct TreeElement *next, *prev, *parent;
 	ListBase subtree;
@@ -58,7 +66,15 @@ typedef struct TreeElement {
 	const char *name;
 	void *directdata;          // Armature Bones, Base, Sequence, Strip...
 	PointerRNA rnaptr;         // RNA Pointer
-}  TreeElement;
+
+	/* callbacks */
+	TreeElementReinsertFunc reinsert;
+
+	struct {
+		/* the element after which we may insert the dragged one (NULL to insert at top) */
+		struct TreeElement *insert_te;
+	} *drag_data;
+} TreeElement;
 
 #define TREESTORE_ID_TYPE(_id) \
 	(ELEM(GS((_id)->name), ID_SCE, ID_LI, ID_OB, ID_ME, ID_CU, ID_MB, ID_NT, ID_MA, ID_TE, ID_IM, ID_LT, ID_LA, ID_CA) || \