Simplify XFCE desktop detection

Current expression doesn't work on my machine by some reason.

```
$ xprop -root _DT_SAVE_MODE       
_DT_SAVE_MODE(STRING) = "xfce4"
$ xprop -root _DT_SAVE_MODE | grep ' = \"xfce\"$'       
$
```

I think we can use simpler expression to detect it that doesn't  depend on grep and works for me.