Issue was with our dear posebones again... when applying overrides we
keep the same address/pointer for the IDs themselves, (which avoids us
the need to remap their usages), but their inner data is often
re-allocated.
Therefore, we need once again to go over armature objects and invalidate
their posebone pointers.
This should also be back-ported to Blender LTS 2.83.
Maniphest Tasks: T80078
Differential Revision: https://developer.blender.org/D8734
'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).
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.
Having functions defined in `BKE_lib_id.h` implemented into
`lib_remap.c` was confusing at best.
Besides trivial code splitting and header includes cleanup, had to add a
new `lib_intern.h` header for callbacks used by both remapping and
deletion code.
