'Private' can be a rather confusing term, especially when considering
its meaning in programming languages.
So now root node trees and master collections are 'embedded' IDs
instead.
The former is always a real, in-Main data-block, while the later, when
different, should be one of those embedded 'private' IDs (like root node
ree or master collection).
Handling those through different ways /might/ be needed sometimes, but
in most case this is just a nest of issues, since you can easily forget
to take them into account.
Note that this should be a 'non-functional' change, as this new behavior
is not used anywhere yet.
Using a struct here allows to change given parameters to the callbacks
without having to edit all callbacks functions, which is always noisy
and time consuming.
