In addition to the "overshoot/bumping into monitor edge" behavior we already
have, we also want to switch pages when simply hovering above the prev/next
page indicators.
This page switch shouldn't happen immediately though, it needs to be kicked
off using a timeout instead. The reason for that is that the next/prev page
indicators are large areas, and simply dragging there isn't enough of a
gesture to really interpret as "the user wants to switch pages".
After this page switch has been toggled once, it can be repeated using the
same "repeat" timeout we introduced with the last commit.
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2581>
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