Make Capture -> Refresh Interfaces work.

Add a "refresh the local interfaces" method to WiresharkApplication,
which reloads the interface list and emits the "interface list changed"
signal, and invoke it from the Capture -> Refresh Interfaces menu item.

Change-Id: I9641e4b341eb44d773c556691d8bb9dac776ea9d
Reviewed-on: https://code.wireshark.org/review/3195
Reviewed-by: Guy Harris <guy@alum.mit.edu>