status/network: Use submenus when necessary

status/network: Use submenus when necessary

We currently stopped using submenus altogether, but that only works

 - if there is a single connection that represents
   the device as a whole

 - if there is just one device, so it is unambiguous
   what device items belong to

To implement that behavior, add a 'single-device-mode' property that
NMDeviceSection can set on its items, and have items update their
'use-submenu' property based on that.

For wireless devices it's a straight mapping, as its items represent
wireless networks that can appear and disappear by just walking around
(multiple wifi adapters also sounds rather fringey).

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2407>