move jakarta-avalon/proposal/* to sandbox

Hi gang,

I almost made this change before and there was basically agreement that
we would make this change, but nicola sort-of objected so now the
proposal dir is still there. It's difficult for people to find stuff
(like berin committed a draft doc and Noel couldn't figure out where).

can we please move this stuff to sandbox? If no objections I'll do it
mid-january or so (as we want to make real sure everyone has ample
chance to object :P).

cheers,

- Leo