Anim: armature edit mode undo now also handles bone collection root count

Armature Edit mode has its own undo handling, which didn't store the
number of root bone collections in the armature. Now it does.

An alternative would be to not store this in the undo step, but rather
loop over all bone collections to find the minimum `child_index`. This
would be linear in the number of bone collections, though, so I chose to
use a few more bytes of memory to make the undo system a little faster.

source/blender/editors/armature/editarmature_undo.cc

@@ -75,6 +75,7 @@ struct UndoArmature {
   ListBase /* EditBone */ ebones;
   BoneCollection **collection_array;
   int collection_array_num;
+  int collection_root_count;
   size_t undo_size;
 };
 
@@ -103,6 +104,7 @@ static void undoarm_to_editarm(UndoArmature *uarm, bArmature *arm)
                                                           uarm->collection_array,
                                                           uarm->collection_array_num,
                                                           true);
+  arm->collection_root_count = uarm->collection_root_count;
 
   /* Always do a lookup-by-name and assignment. Even when the name of the active collection is
    * still the same, the order may have changed and thus the index needs to be updated. */
@@ -137,6 +139,7 @@ static void *undoarm_from_editarm(UndoArmature *uarm, bArmature *arm)
                                                           arm->collection_array_num,
                                                           false);
   STRNCPY(uarm->active_collection_name, arm->active_collection_name);
+  uarm->collection_root_count = arm->collection_root_count;
 
   /* Point the new edit bones at the new collections. */
   remap_ebone_bone_collection_references(&uarm->ebones, bcoll_map);