RNA: add macros for EnumPropertyItem layout elements

Add the following macros for enums as support for these features wasn't
all that obvious and there were some inconsistencies in their use.

- RNA_ENUM_ITEM_HEADING(name, description)
- RNA_ENUM_ITEM_SEPR
- RNA_ENUM_ITEM_SEPR_COLUMN

source/blender/editors/space_outliner/outliner_tools.cc

@@ -2008,7 +2008,7 @@ static const EnumPropertyItem prop_id_op_types[] = {
      0,
      "Remap Users",
      "Make all users of selected data-blocks to use instead current (clicked) one"},
-    {0, "", 0, nullptr, nullptr},
+    RNA_ENUM_ITEM_SEPR,
     {OUTLINER_IDOP_OVERRIDE_LIBRARY_CREATE,
      "OVERRIDE_LIBRARY_CREATE",
      0,
@@ -2067,10 +2067,10 @@ static const EnumPropertyItem prop_id_op_types[] = {
      "Clear Library Override Hierarchy",
      "Delete this local override (including its hierarchy of override dependencies) and relink "
      "its usages to the linked data-blocks"},
-    {0, "", 0, nullptr, nullptr},
+    RNA_ENUM_ITEM_SEPR,
     {OUTLINER_IDOP_COPY, "COPY", ICON_COPYDOWN, "Copy", ""},
     {OUTLINER_IDOP_PASTE, "PASTE", ICON_PASTEDOWN, "Paste", ""},
-    {0, "", 0, nullptr, nullptr},
+    RNA_ENUM_ITEM_SEPR,
     {OUTLINER_IDOP_FAKE_ADD,
      "ADD_FAKE",
      0,