Add a graphical environment to releng

This MR aims to work on issue no #170 and #187.

This MR has an additional profile for a graphical environment (releng-gui), which adds on releng by adding a few other things. These are:

1. The XFCE Desktop Environment
2. networkmanager and applet (working on some of the things mentioned here) edit-done
3. A simple script to setup keyboard layouts
4. Firefox to launch installation guide

The total size of the ISO is 1.2GB.