Work in progress: framework assembly references

The current implementation of framework assembly references don't fit the new LibFolder approach. I tried to re-implement this in a new way. This is not done yet; it's creating invalid XML. I think this is what we still need to implement before we merge:

- merge FrameworkRestrictions for the same assembly path so we get one condition for every path
- group assemblies with the same condition
- handle empty conditions and put them into an "Otherwise" element