Draft: Cellbroadcast support
This adds initial support for displaying cell broadcast messages.
TODOs:
- move MM specific code into PhoshWWanMM so we don't break ofono support
- add a cell-broad-cast-manager that can queue CBMs and triggers the dialog (so PhoshShell doesn't need to bother) (could be follow up)
- use a specific fbd event
This needs the ModemManager side landed first: https://gitlab.freedesktop.org/mobile-broadband/ModemManager/-/merge_requests/1095