There are cases where we want to connect to a number of signals
for the lifetime of an object, but also other signals for a
limited period (say: between show and hide).
It is currently not possible to use disconnectObject() for the
latter, because it will disconnect all signals.
To address this use case, add a small class that can be used as
a transient signal holder, while still benefiting from autocleanup
by proxying the real owner.
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2221>
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