popupMenu: Disconnect signal handlers when child section is destroyed

popupMenu: Disconnect signal handlers when child section is destroyed

Neither `PopupMenuBase` nor `PopupMenuSection` are destroyable, thus the
signal connection needs to be manually disconnected to avoid leaking
both via the respective signal trackers.

This disconnects the connections in both directions when the child
`PopupMenuSection` is destroyed, which handles both cases the section
being destroyed, but also the parent menu being destroyed, because that
also triggers the destruction of all children.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/4398>
(cherry picked from commit 2a923660bc1212cc0249dbb8833d28a708f089c0)