Instead of just a mask of MMModemMode values, we now provide a list of the
allowed and preferred mode combinations supported by the modem. E.g.:

$> sudo mmcli -m 0
  -------------------------
  Modes    |      supported: 'allowed: 2g; preferred: none
           |                  allowed: 3g; preferred: none
           |                  allowed: 2g, 3g; preferred: none
           |                  allowed: 2g, 3g; preferred: 2g
           |                  allowed: 2g, 3g; preferred: 3g
           |                  allowed: 4g; preferred: none
           |                  allowed: 2g, 3g, 4g; preferred: none'