This patch fixes mmcli to set the default timeout on the manager proxy
interface (MmGdbusOrgFreedesktopModemManager1) instead of MMManager
itself. The later is not a GDBusProxy object.
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.