Don't use capture_interface_list() in this dialog.

Instead, if we have no interfaces in global_capture_opts.all_ifaces, use
the refreshLocalInterfaces() method of the main WiresharkApplication to
try to fetch them; that way, if we do find interfaces, a signal will be
emitted to cause the rest of the UI to reflect their arrival.

Change-Id: I59077ef855127fd6bddaa40126c1835a659ffc30
Reviewed-on: https://code.wireshark.org/review/3202
Reviewed-by: Guy Harris <guy@alum.mit.edu>