Include all four desktop environments supported by tasksel (GNOME, KDE,
LXDE and Xfce).
On x86 also add an option in the isolinux menu (under Advanced options)
to select which DE to install; GNOME remains default.
Supports installation of either an xfce or an lxde desktop environment.
For now use B as identifier for INSTALLER_CD in contrib/testingcds.
Includes a framework to manipulate the isolinux configuration for x86 so
that a user can select which desktop he wishes to install.