Add a new class which is going to be responsible for dealing with
ModemManager for us. This is so that:

* We have ModemManager code isolated from rest of the code so its easy to
  add support for other backends (e.g ofono) in future.

* We have only one entity dealing with modem. In the following patch(es),
  we'll drop GClueModemSource baseclass and, GClueModemGPS and GClue3G
  will just use GClueModem for talking to modem.
  * GClue3G can then derive from GClueWebSource instead and be more
    easily modified to submit cell tower data to Mozilla Location
    Service.
  * GClueModemGPS can be easily generalized so that it can be more easily
    modifed to talk to standalone GPS devices too in future (we'll
    probably want to change it's name to GClueGPS then).