XFCE should handle icons automatically... so, you want to be able to see your GTK icon theme in QT applications?
if you want your QT applications to inherit your GTK theme settings, add these two lines to /etc/profile.d/qt4.sh:
GTK2_RC_FILES="$HOME/.gtkrc-2.0:/etc/gtk-2.0/gtkrc"
export GTK2_RC_FILES
log out and in. Tip is from
here.
If it does not work, have a look
here
I actually solved this problem (I was running E17) for me by adding the following in /etc/profile.d/qt4.sh just beneath the ones from above.
export DESKTOP_SESSION=gnome