Jessie desktops

just opening this for review, comments welcome

still 3 major issues left that I'm aware of:

* d-i asks non-preseeded question (rt#3049)
* udev names network interface eth1 (rt#3050)
* puppet-trigger doesn't work on jessie (rt#3052)
Yeah, you're probably right. In theory it's nice to support multiple choices, but in practice I think everyone will just use the default.

Besides, XFCE is vastly superior to LXDE :)