Mpving utils from idcode to idtype proved to be somewhat painful for
some reasons, but now all looks good.
Had to add a fake/empty shell for the special snowflake too,
`ID_LINK_PLACEHOLDER/INDEX_ID_NULL`...
"session-wise" here mean while editing a same .blend file. So creating
or opening a new one will reset the uuid counter. This should avoid any
overflow in practice.
Only IDs added to Main database get an uuid, runtime-only ones are not
affected.
This is intended to provide undo with a way to find IDs across several
'memory realms' (undo speedup project).
No behavior change is expected from this commit itself.
Part of T60695.
Differential Revision: https://developer.blender.org/D7007
