This is only the core extension logic for now, settings UI still needs
to follow.
The logic only acts on the indicator container, not the indicator
itself, meaning that e.g. a screen recording indicator, which is hidden
on the indicator level, can be forcefully hidden, but not forcefully
shown. Because of that and because forcefully hiding it breaks controls
for screen recording, a potential settings implementation should exclude
visiblity controls for some elements like e.g. the screen recording
indicator.
Also handle drag events in this function so connecting to "drag-end" in
widgets with "GtkDragSource" and a `globalThis` `ScrollManager` instance
aren't needed.
Add the `PrefsBoxOrderListBox`es to the `PrefsPage` via the
`PrefsPage`es UI file.
Also let the `PrefsBoxOrderListBox`es initialize themselves based on the
given box order.