Reorganize common code and backend

Since the introduction of DSS contexts, it makes more sense to call CffiApiUtil something based on DSSContext.

The CFFI backend was succesfully split into the `dss_python_backend` package a while ago. Recently, we moved the Obj/Alt API code to https://github.com/dss-extensions/AltDSS-Python

With this, some of the common code was moved from DSS-Python to `dss_python_backend`. It might better to create a separate package, but waiting some time to see if we are wasting too much time handling the common code at `dss_python_backend`, and there is also #2 which might shake things.