wireshark: switch default UI from GTK to Qt

Change the 'wireshark' attribute from 'wireshark-gtk' to 'wireshark-qt'.

In Wireshark 2.x, Qt is the new/preferred UI:

  https://www.wireshark.org/docs/relnotes/wireshark-2.0.0.html

(We keep the explicit -gtk and -qt package variants, as upstream
suggests.)

pkgs/top-level/all-packages.nix

@@ -12570,7 +12570,7 @@ in
   };
   wireshark-gtk = wireshark-cli.override { withGtk = true; };
   wireshark-qt = wireshark-cli.override { withQt = true; };
-  wireshark = wireshark-gtk;
+  wireshark = wireshark-qt;
 
   wvdial = callPackage ../os-specific/linux/wvdial { };