Added support for xfce4-terminal, for Xubuntu.

xfce4-terminal is the default terminal on Xubuntu, gnome-terminal is not installed.
Merge pull request #117 from AdrianSimionov/gns3-gui-0001

Added support for xfce4-terminal, for Xubuntu.
Merge pull request #1 from GNS3/master

Updating my gns3-gui master branch.