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.
Use a consistent style for declaring the names of struct members
in their declarations. Note that this convention was already used in
many places but not everywhere.
Remove spaces around the text (matching commented arguments) with
the advantage that the the spell checking utility skips these terms.
Making it possible to extract & validate these comments automatically.
Also use struct names for `bAnimChannelType` & `bConstraintTypeInfo`
which were using brief descriptions.
