FBX export: Add support for dupliobjects (either children, or particles).

Note only particles/dupliobjects existing in the current frame will be taken into account.

Dev note: the 'object' part of the code becomes really ugly, it will need a serious
refactor (probably using some kind of wrapper around Object/Bone/PoseBone/DupliObject),
nobody (not even me) will be able to follow this code soon.