Add Surface PersistentID

[Reaper90]

For a current project of mine, i wanted to do some calculations based on the model geometry.
Being able to get the PersistentID of a face allows for changes to geometry inSU without losing my surface based calculation parameters, that i need to assign in my project, for existing (but eventually modified) faces.

Thought this could be useful for other user too. I'm not a C++ dev at all, so not sure what other features are possibly affected by my proposed changes. But it works for my purpose of reading a surfaces PersistentID from a SU model.