Windows: Make sure more native dialogs handle HiDPI.

Windows: Make sure more native dialogs handle HiDPI.

Add a WiresharkFileDialog class, which is a thin wrapper around a few
QFileDialog functions that sets per-monitor v2 DPI awareness before
showing native dialogs and resets the awareness context afterward.
Use it where we call QFileDialog::getXXX.

Change-Id: Ib711a70aa94b693a2515804a729f666ea7fbd673
Reviewed-on: https://code.wireshark.org/review/27568
Reviewed-by: Gerald Combs <gerald@wireshark.org>
Petri-Dish: Gerald Combs <gerald@wireshark.org>
Tested-by: Petri Dish Buildbot
Reviewed-by: Anders Broman <a.broman58@gmail.com>