Needed to simplify upcomming fix in Scene undo_preserve code.
NOTE: also renamed 'private' macro parameter names to follow C++ classes
convention (one trailing `_`, instead of one or two leading `_` - the
two `__` leading ones are triggering complains from IDE regarding reserved
identifier names when the relevant macros are used in C++ files).
No expected behavior changes with this commit.
6d1f34a15f