I mean window class manifest data is already parsed in ntdll/actctx.c and FindActCtxSectionStringW() already returns versioned class names.
Good news. I have less work to do.
I mean where in user controls such transition effects are supposed to be used?
Take a look at the default button in Vista and 7. It changes here and there between PBS_DEFAULTED and PBS_DEFAULTED_ANIMATING (the last one not described in MSDN, sorry). If you hover the mouse, the state will be changed smoothly as well. So the push button is the candidate number one. But other controls have animations too, perhaps you didn't recognize it due to a very small transition time.
Speaking about GTK, animated widgets can await you everywhere. For example, in Adwaita (the default theme from GNOME3) tabs have the same behavior as buttons in Windows Vista. For this reason all controls should support animated transitions.