[review] New 'Pco' property in Modem3gpp interface

[Ben Chan]

Hi Aleksander and Dan,

See https://gitlab.freedesktop.org/mobile-broadband/ModemManager/merge_requests/27

This MR adds a 'Pco' property to the Modem3gpp interface for reporting
raw PCO values received from the network as we discussed in
https://lists.freedesktop.org/archives/modemmanager-devel/2018-July/006514.html.
The current implementation supports MBIM modems that implement the
MBIM_CID_PCO notification.  A subsequent MR will migrate the
altair-lte plugin to make use of the 'Pco' property, which will then
allow us to deprecate the SubscriptionState property in Modem3gpp
interface.

Comments are welcome!

For minor issues, please feel free to submit additional cleanup patches.

Thanks,
Ben