Revert "signalTracker: Use WeakMap in SignalManager to avoid leaking objects"

This reverts commit 085102be.

We need the SignalManager map top be iterable at shutdown for some of
the following changes. A WeakMap is not iterable. This revert changes it
back to a regular Map, which re-introduces the leaks caused by this.
Those will be fixed differently by the two followup commits.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2466>