Embedded IDs did not benefit from any of the recent optimizations
(especially for undo case) when writing regular IDs (cleaning up of some
pure runtime data that would generate a lot of fake 'changed on undo'
status).
Now factor out of `write_file_handle` this part of the code generating
temp ID copy with cleaned-up data for writing, and expose it in BLO API
such that IDs owning embedded ones can also use it.
Bastien Montagne