The class only provides the GObject properties that are currently
defined by NMConnectionItem, plus a way to set the active connection
that should be used for the item's :is-active state.
Its sole purpose is to provide a shared base for both device- and
connection items, and to have that base be a GObject so we can
start linking properties via bindings rather than manual fiddling.
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2407>
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