Insure Qt Capture preferences dialog displays default capture device (if any)

Insure Qt Capture preferences dialog displays default capture device (if any)

In CapturePreferencesFrame::updateWidgets() save and restore the default
capture device name because the first call (and only the first call) in
the loop with addItem() triggers on_defaultInterfaceComboBox_editTextChanged()
which unconditionally sets the default name as the first non-hidden device.

Bug: 10965
Change-Id: Ie93f84010a19e8144efa46ce889fb9064979e0e9
Reviewed-on: https://code.wireshark.org/review/9584
Reviewed-by: Gerald Combs <gerald@wireshark.org>
Petri-Dish: Gerald Combs <gerald@wireshark.org>
Tested-by: Petri Dish Buildbot <buildbot-no-reply@wireshark.org>
Reviewed-by: Hadriel Kaplan <hadrielk@yahoo.com>