ModemManager (MM) integration consists of two big parts: things
we do on the networkd start and what we do during the run-time.

Initialization phase
2.1. Wait for networkd to connect to D-Bus
2.2. Setup D-Bus handlers for the essential signals:

• /org/freedesktop/DBus, org.freedesktop.DBus, NameOwnerChanged - to track MM serice availability
• /org/freedesktop/ModemManager1, org.freedesktop.DBus.ObjectManager Interfaces{Added|Removed} -
  to track modem plug/unplug
• /org/freedesktop/ModemManager1/Bearer org.freedesktop.DBus.Properties PropertiesChanged
  to track bearers
  2.3. Check if MM service is yet available: for that call /org/freedesktop/DBus, org.freedesktop.DBus
  ListNames method and see if MM is available. If it is not, then wait for the NameOwnerChanged
  signal and see when it is; finish initialization phase.
  2.4. If MM is available - enumerate modems, see p.4.
  2.5. Finish initialization phase.

Run-time
3.1. During the run-time we track MM service avaialbility. When it is gone we remove all the modems
and bearers.
3.2. When MM is connected we do modem enumeration to get in sync with their current state.
3.3. If a modem was removed we also remove all its bearers.
3.4. If a modem was added we try to start a simple connect.
3.5. If connection was interrupted, e.g. modem changed its network connection from connected state
we start an automatic reconnect.

Modem enumeration proces
4.1. Modem enumeration is done by calling GetManagedObjects.
4.2. By receiving managed objects we try to instantiate all new modems found.
4.3. For that we inspect all bearers available for that modem and add all new bearers found.
4.4. We also read modem ports to detect WWAN interface name assigned to this modem, e.g. "wwan0" etc.
N.B. As we only get the interface name known that late and the corresponding .network file was
already used by the networkd to match interfaces etc. it is not possible
to do things like matching APN to .network and so on.

Simple (re)connect
5.1. Connection is done by calling org.freedesktop.ModemManager1.Modem.Simple Connect method for
the relevant modem.
5.2. It is possible that at the time of connect the operation may fail. For that reason and to ensure
we are always connected we employ a periodic timer which will re-try connection hoping it will
be successful this time or when modem has recovered after an error state and so on.
5.3. networkd will automatically start reconnection if any external entity disconnects modem from
the network.

Open questions
6.1. WWAN interfaces cannot be configured now with the existing .network file:

• ModemSimpleConnectProps - simple connect properties used to (re)connect: I've put
  this under [Network] section for now. Is there a better name for that and location?
• RouteMetric - as we configure address/route dynamically (provided by the modem), we
  do not use any section of .network to define/alter relevant parts. This patch uses
  [DHCP]:RouteMetric for that for now.
• I have a use-case when I do not want the default gateway to be set for WWAN inteface.
  I use [DHCPv4]:UseGateway now which probably needs a better way of defining.
  6.2. Use MM header files - add MM as a build dependency?