rpm-ostree: Improve DBus interface generation and logic

- Use standard Qt types for DBus interface
- Simplify the interface generation logic until we make a full
  abstraction for the rpm-ostree DBus interface.
- Rename the DBus interface generated code to a clearer name.
- Prepare for the removal of the DBus interface internal copy. We will
  be able to switch to upstream definition once the upstream PR
  https://github.com/coreos/rpm-ostree/pull/3112 is merged.