Have the interface "tree" directly connect to ifListChanged.

Then have the capture interfaces dialog connect only *its* code to
update *its* list of interfaces to ifListChanged, rather than connecting
a routine that then emits interfaceListChanged.

This makes it the responsibility of each widget that displays local
interfaces to connect to the ifListChanged signal.

Change-Id: Iff8e8ca4202c8784eed58c8f05175d986a16baea
Reviewed-on: https://code.wireshark.org/review/3153
Reviewed-by: Guy Harris <guy@alum.mit.edu>