Extended Preheat options

Preheat LCD menus base on the code by @D-Base (alternative to pull request #656)

Base on the code by @D-Base, I changed the menu entries the following way:
if only extruder 0 is present and no other extruder or a heated bed: single entry for heating the extruder,
if more than one extruder is present and/or a heated bed, a submenu is displayed containing entries for heating each single extruder, all extruders together and the bed only.

Feel free to deny or to merge. I hope not having offended anyone by issuing this separate pull request.