AdwToasts replaces the GtkLabel inside revealer for notifications
and this simplifies handling the notifications and the toast-overlay
handles the complexities of when to show/withdraw them, the order
they are displayed, etc.
This should significantly simplify these panels, by not forcing
them to override GObject.constructed all the time. Most panels
were quite straightfoward.
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
