Cellbroadcast support

Technically speaking this is not even draft yet.

@devrtz and myself were discussing the lack of CBS support (3GPP TS 23.041) today and had a look at what we already know. @newton688 had some nice logs that I could use to whip up some initial parsing bits. Next up:

• Don't skip initial bytes (We use +cbs instead)
• wire up in receive path (after figuring out how much overlap there is with SMS)
• make it proper GObjects (hints on what we want as properties welcome)
• add api to enable/disable cbs? (follow up)
• api to set channel list
• read channels from usim (efbcmi) (can likely be follow up)? (follow up)
• expose cbm on DBus (e.g. /org/freedesktop/ModemManager1/CBM/0)
• expose more CBM properties
  • identifier
  • message code or popup / alert
• handle coding scheme (ETSI TS 123 038), (we currently assume 7bit GSM)
• See TODO in last commit
• mm-glib (could be follow up?)
• mmcli support
• load channel list on startup (can be follow up?)

This is very much modeled like SMS but without the sending/composing bits.

Targeting #253