The _devices property is another case of overloading the
"device" term.
Fun fact:
this._devices[device._delegate.category].devices
uses three different meanings of "device" (section, NM.Device, item).
The devices array in sections won't be around for much longer,
but the property that tracks the sections is worth renaming.
While at it, use a map instead of a plain object, which has a
guaranteed order when iterating (which will come in handy later).
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2406>
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