UI: Add missing icon to Move to Bone Collection

Add the "plus" icon to the "New Bone Collection" operator,
and the separator line before the list of bone collections.

It now matches the object's "Move to Collection" operator, and
Grease Pencil's "Move to Layer" operator.

source/blender/editors/armature/bone_collections.cc

@@ -1070,12 +1070,14 @@ static void move_to_collection_menu_create(bContext *C, uiLayout *layout, void *
   uiLayoutSetOperatorContext(layout, WM_OP_INVOKE_DEFAULT);
   uiItemIntO(layout,
              "New Bone Collection",
-             ICON_NONE,
+             ICON_ADD,
              is_move_operation ? "ARMATURE_OT_move_to_collection" :
                                  "ARMATURE_OT_assign_to_collection",
              "collection_index",
              parent_bcoll_index);
 
+  uiItemS(layout);
+
   /* The remaining operators in this menu should be executed on click. Invoking
    * them would show this same menu again. */
   uiLayoutSetOperatorContext(layout, WM_OP_EXEC_DEFAULT);