Blender Git Loki

ID Duplicate: Factorize a lot the code.

Now that we have a uniform consistent behavior in all our ID duplicate
funtions, we can easily factorize it greatly. Code gets cleaner,
smaller, and less error-prone.

Note that ultimately, this duplicate/deep copy behavior could be added
as a callback of IDTypeInfo.
We could also rethink the duplicate flags (some data, even some obdata,
like Lattice, are not coverred currently).
And so on. But at least code should now be much more easily maintainable
and extendable.