The RouteGroup containing the deleted routes emits the RouteRemoved signal; Session handles this
and if the RouteGroup is now empty, deletes the RouteGroup *while in the middle of the signal
emission process*. This deletes the Signal (since it was owned by the RouteGroup) which leads
to a crash inside the signal emission code, since that is now running "on" a deleted Signal
object.
This change simply makes that signal static, and so deleting the RouteGroup has no effect
on the Signal itself.
More changes to come related to better (shared_ptr<RouteGroup>) mgmt of RouteGroup lifetimes.
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