xsession: use gtk3 and cleanlooks for Qt

Qt 5.7 now includes support for gtk3 instead of gtk2. gtk2 has moved to
qtstyleplugins. The Debian package however doesn't (yet?) include a
version with gtk2. There isn't a gtk3 style (packaged?), either.

So, I for now will use the gtk3 platformtheme and the cleanlooks style.
In fact, it doesn't change the appearance too much, so I might even
stick to it.

.xsession

@@ -21,8 +21,8 @@ if [ "$(hostname)" = "kevin-desktop" ]; then
 fi
 
 # Use GTK style in Qt5 applications
-export QT_STYLE_OVERRIDE=gtk
-export QT_QPA_PLATFORMTHEME=gtk2
+export QT_STYLE_OVERRIDE=cleanlooks
+export QT_QPA_PLATFORMTHEME=gtk3
 
 # Disable overlay scrollbar in GTK 3
 export GTK_OVERLAY_SCROLLING=0