Adds xfce4-terminal as an option

Because I like to use it, and XFCE is included in one of the main Ubuntu distributions, Xubuntu.

I foresee future requests for addition of the following terminals:

- aterm
- guake
- lxterminal
- rxvt
- terminator
- tilda
- urxvt
- yakuake

If you'd like I could add those, but I'm not familiar with them.