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.