GTK has deprecated the AppChooser interface and related widgets.
These APIs will be gone in GTK5, so we should move away from
them as soon as possible.
This MR takes the opportunity to change the design to match newer
mockups while dropping deprecated APIs.
This avoids coggling the UI for those devices for which a modem
is not present, as x-scheme-handler/tel and x-scheme-hanlder/sms
are of no use in such cases
These labels were introduced in an ancient time (pre 2011) when
the panel still consisted in a table. They are unnecessary
and have no UI impact at this point