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.)