Eventually each section will turn into its own menu, which should
include a single settings item.
This also removes the odd duplication of items, now that we avoid
using submenus where possible.
In general this is straight-forward, except for modems: Some
models are now supported by a dedicated wwan panel, while others
still use the generic network one.
Address this by adding items for either panel, but only show one
at a time. The new panel is used if *any* modem is supported,
only when all modems require it, the legacy panel is used.
Hopefully that shouldn't be an issue for many users, as using many
different modems with different capabilities should be fairly rare
(except for Aleksander Morgado, but I think he can handle it)
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2407>
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