LibOverride: Also consider regular embedded IDs as virtual overrides.

Those (aka root node trees and master collections) share same 'traits'
as the shapekeys: RNA cannot assign them, so we need to handle them as
sub-data of their owner IDs.

Not sure how much this will help in improving support of overrides for
scenes and nodetrees, but this is a mandatory step towards that goal for
sure.

source/blender/makesdna/DNA_ID.h

@@ -515,7 +515,7 @@ typedef enum ID_Type {
    ((const ID *)(_id))->override_library->reference != NULL)
 
 #define ID_IS_OVERRIDE_LIBRARY_VIRTUAL(_id) \
-  ((((const ID *)(_id))->flag & LIB_EMBEDDED_DATA_LIB_OVERRIDE) != 0)
+  ((((const ID *)(_id))->flag & (LIB_EMBEDDED_DATA_LIB_OVERRIDE | LIB_EMBEDDED_DATA)) != 0)
 
 #define ID_IS_OVERRIDE_LIBRARY(_id) \
   (ID_IS_OVERRIDE_LIBRARY_REAL(_id) || ID_IS_OVERRIDE_LIBRARY_VIRTUAL(_id))