Again no big surprise, a property that will soon correspond to the
quick toggle's icon.
It is more involved than the :checked property though, which is just
a simple boolean.
To keep it as simple as possible, we set up a binding group and use
that to bind the :icon-name property.
As state changes, we then update the group's source with the item
we deem to best represent the section as a whole at the given
moment.
That is (in that order):
- the first active item
- the most recently used item
- the top-most visible item
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2408>
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