The panel supports 2G/3G/4G GSM/LTE modems. CDMA2000 Modems are not supported.
If a supported modem is present, the panel will be shown and the modem will be
handled, else, network-panel shall manage the modem as it did in the past.
If more than one modem with data enabled is present, the user is allowed to set
priority of one SIM over the other (the priority is for SIM, not modem).
Fixes https://gitlab.gnome.org/GNOME/gnome-control-center/issues/132
