Start using the new methods to simplify signal cleanup. For now,
focus on replacing existing cleanups; in most cases this means
signals connected in the constructor and disconnected on destroy,
but also other cases with a similarly defined lifetime (say: from
show to hide).
This doesn't change signal connections that only exist for a short
time (say: once), handlers that are connected on-demand (say: the
first time a particular method is called), or connections that
aren't tracked (read: disconnected) at all.
We will eventually replace the latter with connectObject() as
well - especially from actor subclasses - but the changeset is
already big enough as-is :-)
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1953>
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