Avoid trying to merge all interfaces into the same set of APIs, and keep having
a specific object (proxy) for each interface handled by the GDBusObjects
reported listed by the GDBusObjectManager.
The library will contain a small layer of code on top of the proxies and object
manager client generated by gdbus-codegen. Therefore, the headers auto-generated
from the DBus introspection XMLs need to be distributed along with the
libmm-glib specific headers.
