CHOP-854: move change_(item|category)_parent() into LLInventoryModel.

It's not really clear why they're in llinventoryfunctions.{h,cpp} instead of
LLInventoryModel, and in fact LLInventoryModel::removeItem() already contains
code essentially cloned from change_item_parent() -- or perhaps vice-versa.
This changeset addresses only migrating the functions, and fixing up existing
references, to simplify code review.