signalTracker: Add TransientSignalHolder class

signalTracker: Add TransientSignalHolder class

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>